Allow to patch modules imported with another name

[mrbean-bremen]

This is related to #191.
tempfile imports os as _os and io as _io. With the current mechanism, these will not be patched by Patcher.
For this and other cases with custom modules it makes sense to have a possibility to add these to the patched names, for example by providing an additional dict to Patcher (and TestCase) that holds these names (e,g, something like {'tempfile': {'os': '_os', 'io': '_io'}}).
This could be pre-configured for tempfile, but have the possibility to add other modules.