Add custom Angular app folder. Cleaned up version of #148

This is the cleaned version of #148. All commits & final merges squashed into 1. 
Major change logs are in [this comment](https://github.com/Swiip/generator-gulp-angular/pull/148#issuecomment-65871379).
The appPath is added as the first question, with default at `src`, can support multiple level of folder like `path/to/src`.