DRY, use `inspect.signature` for `curry`?

[peteut]

Hi,

I did some experiments w/ inspect.signature and it works quite well. curry.py gets much simpler, but there is no checking for updated **kwargs in yet.

Cheers, Alain

[jackfirth]

Is it possible to use it in 2.6 and 2.7? I don't intend to drop support for those versions.

[peteut]

CI only checks from 2.7 upwards. I'll add checks for 2.7 in that case.

[peteut]

Added 2.6 tests, works w/ future.moves. funcsigs is a dependency in addition. Travis CI tests can be found here.