Add new API: jedi.api.get_definitions

From docstring: This functions can be used for listing functions, classes and data defined in a file.  This can be useful if you want to list them in "sidebar".  Each element in the returned list also has `get_definition` method which can be used to get sub-definitions (e.g., methods in class).

I will need to add something like `Definition.name` otherwise there is no way to get class/function/variable name without touching internals.  But before doing so, I wanted ask if this API is good for jedi.

BTW, for consistency with `Completion.word`, is `word` better than `name`?