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

| File Type         | File Count       |
|:------------- |-------------:|
| test | 1250 |
| example | 250 |
| .npmignore | 240 |
| .travis.yml | 150 |
| .h | 150 |
| build | 150 |
| default_app | 65 |
| benchmark | 50 |
| .cc |  40 |
| unused fixtures | 15 |

This PR on Mac OS X: **1,916 directories, 6,011 files**