Adding a little command line stuff for windows support in lein-npm and lein-bower

Basically I just made a few changes so that bower would be able to pull in dependencies.  I only found lein-npm a couple days ago but I figured it would be a good as place as any for my first pull request! (be gentle!).  

If you have any questions on any of the changes let me know.  I tested the changes by running lein install for my local clone and using lein npm and lein bower to install some dependencies into my dev directory.  Needless to say it worked, and that is awesome.