Rewrote the fowards parameter howto in a separate section, added an 'autosignature' directive in sphinxext
Documented specifiers.forger_function and set_signature_forger. Also allowed arbitrary wrappers to be used with the latter.
Switched from _util.signature to specifiers.signature where applicable.
Renamed sigtools.specifiers.forwards_to to forwards_to_function
Fixed signatures.merge not propagating the change to previous parameters when marking a parameter as positional-only
Updated example output to python 3.4 format.