Merges pull requests 4 (by leanucci) and 6 (by cav21).

- Implemented basic access to the collection search method;
- Changed the test credentials;
- Added documentation for collection search;
- Using Ruby 1.9 hash format;
- Version bump.
Adds refresh_access_token to the authentication module.
Added credits to Readme
Request uses Faraday instead of RestClient for better SSL support.
Updates README.md
Merge remote-tracking branch 'upstream/master'

Conflicts:
	README.md
	lib/mercadopago/version.rb
	test/test_mercado_pago.rb