Add support for plugin symbolic links

Useful in when developing a plugin, changes support both adding and uninstalling plugins.
I implemented the option for plugman because it didn't work there also. Here's my pull request which also updates the shelljs library version: https://github.com/apache/cordova-lib/pull/85. 
I verified that all unit tests pass and that it doesn't delete the target directory of a symbolic link - so you can safely remove this code, afterwards.