Simplified Travis test script and allow coverage views at https://coveralls.io

Instead of having nosetests call coverage, it is much more clear to have
coverage call nosetests.  We still require 100% coverage of the module (not
tests) to pass.

More tests were add so "pypy" version passes coverage.

It is really, really easy to set up https://coveralls.io .  It is just like
TravisCI: simply login with your github account, give coveralls.io permission,
then enable the repo.

Added coverage badge from coveralls and version badge from https://pypip.in

See #70
curry raises error on non-callable input
explicitly test _num_required_args
Merge pull request #1 from mrocklin/coveralls_for_master

curry raises error on non-callable input
Skip coverage for pypy
Add downloads badge to README
Merge pull request #81 from eriknw/coveralls_for_master

Simplified Travis test script and allow coverage views at coveralls.io