Added support for relative paths for :bower_target_path

I've added support for relative paths in :bower_target_path. 

Having it support only absolute paths doesn't quite work, since the value of _release_path_ (as in the example from the readme) changes during execution it is handy to just specify the relative path so that the correct value of _release_path_ is fetch when bower:install is invoked.

Since I am not a ruby developer I am sure there are better ways to achieve this. Feel free to reject the pull request and suggest a better way, I will try to fix it and resubmit as needed.