On Red Hat, CentOS and Fedora systems, the pip binary will be called python-pip
instead of pip. This commit makes the pip module also check for python-pip.

The reason we check for python-pip *first*, is to have ansible fail on not
finding 'pip' and reporting *that*. This is consistent with current behaviour
and will not confuse users of Debian et al., where the 'python-pip' binary
never exists.

Tested on Fedora 18 and Ubuntu 12.04.