Enable fixture inheritance

Enables fixture inheritance as described in https://github.com/nelmio/alice/pull/46, with the suggested syntax by @Seldaek on the comments https://github.com/nelmio/alice/pull/46#issuecomment-21555671


I don't mean to take over the work of @erichard done in that branch, but there were no recent updates (8 months for the last update, as of this PR). We had to implement it for our fork, so decided to contribute it back with documentation and tests.

Closes https://github.com/nelmio/alice/pull/46
Might also be related to https://github.com/nelmio/alice/pull/28