use unittest's assertRaises implementation if it exists
Test custom emulation class for specifiers
Added deep attribute support for forwards_to_meth
Test __wrapped__ correctness for _ForgerWrapper instances
Allow for forger functions to return None in order to keep the default signature.
Add explicit dependencies in tox.ini as well as a coverage testenv.
Fix .gitignore
Moved dep-less sigtools.signatures innards to _signatures, fixed sig of partial objects.