fix dump_module() bugs and rename parameter 'main' to 'module'

[leogama]

Fixes #525

New phrasing of mismatching modules error messages in load_session():

>>> import dill
>>> dill.dump_module()
>>> dill.load_module(module='math')
ValueError: can't update module 'math' with the saved state of module '__main__'

>>> import types
>>> main = types.ModuleType('__main__')
>>> dill.load_module(module=main)
ValueError: can't update module-type object '__main__' with the saved state of imported module '__main__'

>>> dill.dump_module(module=main)
>>> dill.load_module(module='__main__')
ValueError: can't update imported module '__main__' with the saved state of module-type object '__main__'

[leogama]

@mmckerns Job finished here.

[mmckerns]

LGTM. There are a very few doc updates that I'll make post-merge.