Adding REST methods for ensembles

- REST api methods for ensembles
- Improving error handling in get methods
- New method returning the leaves of a model
Improving and relocating some functions and typos
Adding new wrapping function for dir assignation

- Adds a new wrapping function for the storage directory assignation
- Changes check_resource and get_resource_id methods to functions
- Improves code in error message handling
Bug fixing: changing data to body to retrieve request data in requests lib
Adding function to retrieve the locale used in bigml.com from the user locale
Bug fixing: internal _error_weight key is now removed after use
Bug fixing: typo in reference to api object in ensemble
Version bump to 0.6.0