brooklyn launch vs. spaces\ with\ paths

Wrapped variables in brooklyn.sh to avoid errors when the path contains spaces.

Sample error case, having followed the Getting Started instructions:
```
$ pwd
/Users/username/Brooklyn Runtimes/brooklyn-0.7.0-SNAPSHOT
# NB: space in path ----^

$ brooklyn launch
usage: dirname path
usage: dirname path
/Users/username/Brooklyn Runtimes/brooklyn-0.7.0-SNAPSHOT/bin/brooklyn: line 34: //pid_java: Permission denied
Error: Could not find or load main class brooklyn.cli.Main
```

TODO:
* Alter Getting Started instructions to set paths using `BROOKLYN_DIR="$(pwd)"` (with quotes).