Add example Gemfile entry to README

The gem name doesn't match the repo which might be confusing to people, especially since 'oauth2-provider' is a real (different) gem.
Updating to remove deprecation warnings when using ActiveRecord 4.
Checking for ActiveRecord::VERSION only if it has been defined.
Appraising with ActiveRecord 4 only if Ruby verison is >= 1.9.
Using Ruby 1.8.7 compatible hashes.
Adding a Helpers module containing ActiveRecord-version-agnostic 'count' method.
Replacing duplicate travis build entry of 1.9.3 MySQL AR3.2 with AR4.
Fixing wording of contexts in Helpers module spec.
Fix rspec deprecation warnings.
Merge remote-tracking branch 'harigopal/activerecord-4'
Update various build config files.
Annotate .travis.yml.
Merge pull request #51 from lann/patch-2

Add example Gemfile entry to README
Replace Bundler instruction with gem-install instruction.
Add mysql to the Rails 4.0 gemfile.
Remove all the trailing whitespace.
Do not allow the user arg to Provider.access_token() to be nil. If you want implicit user lookup, pass :implicit as the user. This prevents accidental authorization in cases where the application developer does not check the result of their User.find() calls.
Merge branch 'master' of https://github.com/songkick/oauth2-provider into upstream-master

Conflicts:
	lib/songkick/oauth2/model/authorization.rb
	lib/songkick/oauth2/model/client.rb
	lib/songkick/oauth2/provider/exchange.rb
	lib/songkick/oauth2/schema/20121025180447_songkick_oauth2_schema_add_unique_indexes.rb
	spec/songkick/oauth2/provider/exchange_spec.rb