yml support for configuration files

[davidemoro]

Yaml will become the preferred serialization format instead of json (perhaps the unique serialization format if we decide to drop the json format).

Pro:

• more user friendly
• no brackets
• less prone to errors (compared to json trailing commas for non technical users)
• always parseable by default ({"property": $something} is not parseable and no easy way to implement a validator, where for $something we could have 1, "a string", null for example). property: $something can be validated

Cons:

• a large amount of documentation (and third party plugins documentation) will need to be updated
• in case in future releases we'll decide to drop json, no more backwards compatible (any strong opinion for maintaining json?)
• play_json fixture soft deprecation and fixture rename

[davidemoro]

not yet requested by any user, close

[davidemoro]

Considered useful by users, reopen

[davidemoro]

Feedback from users: deprecate json and use yaml.
play_json fixture deprecated and it will removed in version >=2.0.0

[davidemoro]

Merged in features branch, waiting for new 2.0.0 release