added units example to docs

This was causing unintended resuce from other exceptions like:
Errno::ECONNREFUSED: Connection refused - Unable to connect to Redis...

rack/utils is not a dependency and therefore cannot be required.

This was causing an exception when `join` was called on the resulting
string.

Add Memcached cache client.

Make Geocoder.cache return Geocoder::Cache instance instead of underlying cache store.

Substate for Google result

instead of inferring from response length. This is less than ideal but
it fixes issue #396 and, given what appears to be MaxMind's inconsistent
response lengths, it may not be possible to do much better.

Add support for setting a default stub in test mode.

Update README regarding unknown coodinates.

Tweaks to wording in section on forward and reverse geocoding for coordinates that do not have known location addresses associated with them.

Fix deprecation warnings on active record scopes

In addition to checking for []/[]= and set/get, also check
for functions like read/write.

Yahoo auth errors: add raw message to warnings