Update gemspec: summary, description, and files. Also add json gem
dependency and update CHANGELOG and README with lots of new features!

Does not seem like a significant gain to have the auto-assignment AND a
custom block. In fact, it seems annoying to have to use a different
method. So now, passing a block to geocoded_by or reverse_geocoded_by
prevents any auto-assignment from happening.

Use 'geocode' instead of 'fetch_coordinates!'.

Should have been part of commit cbda51e8.

Detect this at the Lookup level (instead of the Rack::Request level, for
example) and return the response we know Freegeoip.net will give.

This should have been part of commit 8cc460f9.

Add geocode and reverse_geocode methods along with bang versions
which can be used to skip auto-assignment of address or coordinates
(specify a block that handles the Geocoder::Result instead).

Be sure to return nil.

If forward and reverse geocoding were both declared and options passed
for both, only the options for the first were being set.

Kill some old, unnecessary require statements.

More than one result is very rare and it's not clear how one would
evaluate the choices. If this is required in the future a new method
can be added.

This fixes a scope problem: blocks are evaluated in the context in which
they are declared so custom routines were being evaluated in the context
of the class, not the object.

Don't take 'reverse' argument, instead figure it out by looking at what
options were specified for the model. Also, don't raise exceptions on
bad configuration (no longer possible to detect, wasn't that important).

city, country, country_code, postal_code

Should have been part of commit 29af848a.

Split result parsing into separate method.

No longer necessary.

Use JSON (json gem) instead.