Update readme and configure CI.
Set default rake task to 'spec'.
Fix typo in readme.

[ci skip]
Added appraisal gem as a development dependency.

Also set up appraisals for ActiveModel versions 3 and 4.  Appraisal's
generated gemfiles added to Travis config.  Rakefile also configured per
Appraisal instructions and all gemfile.lock files ignored.
Configure Coveralls.

Also add badge to readme.
Add Gemnasium badge.

[ci skip]
Fix quoting style.
Add ActiveModel support for states and counties.

This will only kick in if ActiveModel is already defined (i.e. in a
Rails environment) *before* this gem is loaded.
Modify the API.

State is no longer a member of county, but its ANSI code instead.
Counties still have a state accessor.  County members are renamed
'state_ansi' and 'county_ansi' to line up with the original file.
This is pretty arbitrary, but now states and counties are more
independent.
Update tests to reflect API changes and ActiveModel support.
Merge branch 'feature/active_model_support' into develop