Generating hsfiles file with the --hsfiles switch in yesod init

This patch creates a new file "HsFile.hs" in yesod-bin, and adjusts main.hs to allow the --hsfiles flag in yesod init. 
The HsFile module goes through all the files in the current directory and subdirectory and returns a list of FilePath that is consumed by the createTemplate, this is my first try with conduits so the code is prone to heavy adjustments.

I made some simple tests with some folders and yesod init managed to reproduce them well.