Build script improvements

Fixes adobe/brackets#2583

* Add optional `-installer` (or `-i` for short) parameter for `build.sh`. If this parameter is set, the build will be staged and installers created (mac only, for now...).
* Make `NO_FETCH` the default branch for brackets and brackets-shell. 

The net effect of both of these changes mean that simply running `scripts/build.sh` will _only_ build the appshell, and will not do any staging for the installer. This also means that your local source (or branch) will _not_ get clobbered unless you explicitly set the `BRACKETS_BRANCH` or `BRACKETS_SHELL_BRANCH` variables. 

To build installers, call with `-i` or `-installer`:
```
scripts/build.sh -installer
```

You only need to set the `BRACKETS_SRC` variable if the `-i` parameter is passed.