Load single fixture file (issue #36)

Early implementation of requested feature *Load single fixture file* (issue #36). Main changes:

* The command line has a new `--fixture-file` option with a required value: one bundle fixture file that **must** be defined within khepin_yaml_fixtures.resources section. If not, it will throw an \Exception().
* Now `YamlLoader::loadFixtures` and `YamlLoader::loadFixtureFiles` may receive one extra argument (the reqiured value specified with --fixture-file option)
* Redefinition of YamlLoader::bundles class property just with the specified single bundle fixture file.

About the exception thrown if an unknown bundle fixture file is specified: I really don't know which is your best choice for that, but in my opinion when a user write a incorrect value for --fixture-file option, the execution shouldn't countinue. And an exception seems (for me) the best choice.