yum url support

Hi,

I'd like to propose a patch for the yum module to avoid stacks like this when a URL is provided rather than package name/path:
```
msg: No Package file matching 'https://rhuiqerpm.s3.amazonaws.com/python-coverage-3.6-0.2.b3.el6.x86_64.rpm' found on system
```
The patch goal is to try handling the rpm spec as if it was a URL in case it wasn't found on the system. It adds a dependency on urllib.

Thanks!