Remove unneeded files from distribution

After the Windows release, there were several comments/tweets about how many files are in the Atom app distro and it taking awhile to unzip.

This PR takes a look at what files are currently included and removes files that aren't actually needed to run Atom. The result being a smaller download and a faster install.

### Before

Version 0.115.0 on Mac OS X: **2,535 directories, 8,388 files**

### After

| Type         | Count       |
| ------------- |-------------:|
| test files | 1250 |
| example files | 250 |
| .npmignore files | 240 |
| .travis.yml files | 150 |
| .h files | 150 |
| build files | 50 |
| .cc files |  40 |

**2094 directories, 6893 files**