Prefs

- Changed all function names accord to PEP8:
* `PREFS.ReadPrefs` -> `PREFS.read_prefs`
* `PREFS.WritePrefs` -> `PREFS.write_prefs`
* `PREFS.OverWritePrefs` -> `PREFS.overwrite_prefs`
* `PREFS.ChangeFilename` -> `PREFS.change_filename`
* `PREFS.ConvertToJson` -> `PREFS.convert_to_json`
* `ReadJsonFile` -> `read_json_file`
* `ReadPREFSFile` -> `read_prefs_file`
* `ConvertToPREFS` -> `convert_to_prefs`

- Added `PREFS.convert_to_yaml` and `read_yaml_file`.

- Fixed bug when creating a pref with an empty dictionary as value.

- Added `ConvertToPREFS()` function (outside PRFS class) that do the same as `dumps()` in json. Converts a dictionary into a PREFS file but instead of writing the PREFS in a file returns it as string.
- Changed `ReadPrefs()` function name (outside PREFS class) to `ReadPREFSFile()`.
- Fixed issue when dictionary=True parameter, fixed issue when dictionary=True and interpret=True parameter.

- Added ReadPrefs function outside PREFS class that reads a PREFS file and return it's value.

- Fixed error when overwriting prefs and using a lambda function.

- Added tree/cascade (nested dictionaries) support.
- Added docstring.