Stricter folders and files structure

[javierbrea]

Currently mocks-server supports autodetection of fixtures and behaviors inside all subfolders and files in the mocks folder. This may produce errors and undesirable effects, and it is a kind of "magic". It also require fixture handlers to implement a mechanism to detect fixtures which should handle, etc.

In order to promote best practices, avoid package complexity and possible bugs, it is desirable to add an option for defining the folder from which the fixtures will be loaded, and the folder or file from which the behaviors will be loaded (probably a single file better). Fixtures should also include an identifier about the fixture handler to be used to load it, so handlers should not provide a function to let the mocks-server know it an object is desired to be handled by one or another.