added test (stubs) for two more modules

I added test (stubs) for ``lingpy.basic.tree`` and ``lingpy.basic.parser``. In particular ``lingpy.basic.parser`` contains some rather difficult to test code. The various ways in which the input for a ``Parser`` instance are looked up should be unified and factored out into some sort of input lookup function which can be shared across modules. In particular the implicit cache protocol - i.e. looking for pickle files in an (AFAIKT undocumented) ``__lingpy__`` directory (relative to the current working directory) - should either be centralized and documented or thrown out.