Modernized the gem, and a few other tweaks.

Now it looks like what a gem generated with `bundle gem` would look
like.

Also:
- Change requirements back to ActiveRecord 3.1, seen as the gem is not
  ready for prime time with Rails 4;
- Silence tests;
- Indicate which license this gem has.
Support AR from 3.0 onwards.
Merge pull request #1 from andremedeiros/modernize-gem

Modernized the gem, and a few other tweaks.
Added gemfiles to test AR 3.1 and 3.2
Make Travis test only the versions that we actually support
Lock sqlite3 version so tests run with rbx
Silence more deprecations
Documentation improvements

- Add build status badge
- Mention that the gem also works for Rails 3.1
Move the AR dependency back to 3.1
Add Zachary to the authors and change the gem homepage.
Minimal amount of work to make tests run with AR 4

Right now, there are a lot of deprecation warnings and a couple of
failing specs.

Run with:
`BUNDLE_GEMFILE=gemfiles/active_record_40.gemfile rake test`

This commit also adds an extra development dependency for the
`rails-observer` gem, so that we run the exact same specs than the 3.0
branch.
Make tests pass on AR 4.