exec functions

Created by: sajattack

This isn't really ready for merge yet but I want to open this up for discussion. In issue #64 (closed), it was mentioned that we don't want C functions in our header files, but we need some intermediary C to interact with va_lists and char**s. I tried to implement execvp without header C, as you will see commented out, but couldn't get it working. I'm open to suggestions.