Fix #273 add optional --offline flag for fast recompile

Adds an offline flag to be invoked as:

```
rake -- --offline
```

This skips things like Git submodule init and update calls, as well as jQuery remote download. In the case of Git submodules, it simply logs a notice (although the compile will fail if the submodules have never been initialized), whereas, in the case of jQuery, if it's trying to remote download it, then it definitely isn't already present, in which case it will throw a hard fail.