support for opencagedata optional parameters

Don't assume mapbox results have context

* Refactor calculations

Refactored calculations::extract_coordinates and calculations::coordinates_present?

coordinates_present?
Checks whether an argument matches validating criteria (numeric, not NaN), rather than returning false when matching invalidating criteria.

Also replaced .to_s == NaN with .to_f..nan? 

extract_coordinates 
Now calls coordinates_present? to validate the array, instead of comparing four booleans and initialising extra vars.

* Removed .map and ternary from coordinates_present?

Updated pull request according to Lime's suggestions.

Update README.md

[TYPO] Fixes a couple of typos in the README

Fix README regarding dstk :host configuration

Add Lookup::Test.reset to README 

The method `#reset` for clearing stubs
was previously unmentioned in the README

:host must be nested within the :dstk key
in order to be recognized.

Added LocationIQ provider

LocationIq # for classes
location_iq # for config

No escaping should be necessary, according to the MaxMind docs:
http://dev.maxmind.com/geoip/geoip2/web-services/

Make geocoded? return false with single coordinate

support canada for geocodio

Use sqlite ruby math extensions if present

Add test coverage for LocationIQ lookup

Fix result of ipinfo.io for loopback address (127.0.0.1)

Prevent rake task crash when BATCH not specified

In Rails 5, `rake geocode:all` crashes:

```
$ rake geocode:all CLASS=Whatzit
rake aborted!
ArgumentError: comparison of Fixnum with nil failed
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:217:in `<'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:217:in `block in in_batches'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:198:in `loop'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:198:in `in_batches'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:120:in `find_in_batches'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:58:in `find_each'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/geocoder-1.2.2/lib/tasks/geocoder.rake:11:in `block (2 levels) in <top (required)>'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/rake-11.2.2/exe/rake:27:in `<top (required)>'
/Users/paul/.rvm/gems/ruby-2.2.2/bin/ruby_executable_hooks:15:in `eval'
/Users/paul/.rvm/gems/ruby-2.2.2/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => geocode:all
(See full trace by running task with --trace)
```

The problem seems to be that `find_each` used to default the `batch:` param when it was explicitly set to nil, but now passes an explicit nil on through.

This patch fixes the issue by defaulting the batch size in the rake task.

Use sqlite_ext 1.5.0 which has been broadened to work with Ruby
versions prior to 2.x, and remember to add it to files in the
./gemfiles/ directory for use with CI tests, not just to
Gemfile in the root directory.