Build script improvements

Fixes adobe/brackets#2583

* Add `BUILD_INSTALLER` option for `build.sh`. If this variable is set, the build will be staged and installers created (mac only, for now...). This variable is *not* set by default.
* 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. 

You only need to set the `BRACKETS_SRC` variable if the `BUILD_INSTALLER` variable is also set.