Bump to version 0.0.6

- Add Rspec to Rakefile
- Update Default Extractors List
- Refactoring of assign_request_options which now uses an Array that lists authorized options.
- Add 'language' to the list of authorized request options
- Refactoring of Request.build_query + Add support for language
- Refactoring of Entity (underscore string transformation for the attribute names mapping)
- Implement Entity#freebase_id

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>