fix dump_module() bugs and rename parameter 'main' to 'module' (#526)

* fix dump_module() bugs and rename parameter 'main' to 'module' (fixes #525)

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

```python
>>> 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__'
```

* dump_module: clarify refimport description

* improvements to 'refimported' handling and extra checks in *_module() functions

* load_session(): clarify that the 'module' argument must match the session file's module

.gitignore

@@ -5,4 +5,4 @@
 /docs/build
 /build
 /README
-/dill/info.py
+/dill/__info__.py