Drops Narwhal support

* `require("ringo/engine").getRingoHome().getPath()` instead of `require("system").prefix`
* Changes use of RINGO_HOME in `ringo-admin install package`
* `/bin/ringo` drops narwhal option which requires legacy mode
Fixes import of ringo/engine
Merge pull request #263 from botic/drop-narwhal

Drop narwhal, fixes #261