Enable loading fixtures from arbitrary locations

This patch enables loading fixtures from subdirectories of test/fixtures.

This allows to have two or more sets of fixtures in a project - very helpful if someone needs a lot of typical "lorem ipsum" fixtures (i.e. when developing layouts) while others need compact fixtures to be used solely for tests.

You can rake db:fixtures:load FIXTURES_DIR=real and expect to test/fixtures/real/* to be loaded.

This patch has no tests - I'd be happy to write them if someone points me in the right direction (are there any tests for rake tasks in Rails?) My team was using this patch for last few months and we have had no problems at all.