mkrpm grunt task

This pull request is referring to [issue #301](https://github.com/atom/atom/issues/3011)

The "mkrpm" grunt task will create the rpm package in red hat Linux (Fedora, CentOS, Red Hat)

This assumes that the build task is **done**, and the rpm build tools are installed,
which can be done with the yum package manager:
* sudo yum install @development-tools
* sudo yum install fedora-packager

**NOTE**: I have only tested in the Fedora-20 machine, but it should work in
CentOS and Red Hat Linux as long as the rpm build environment is installed.