Added node_modules path reject to SlnBuilder build method.

Hi Amir. I was running into an issue where my binaries in my node_modules were disappearing and I kept having to reinstall them. I tracked it down to the /bin & /obj cleanup in sln_builder. 

Not sure if this is the most flexible solution, or if there might be a more flexible way of verifying that you are in a project directory before running the cleanup (rather than adding a reject for every directory that might have /bin or /obj).