Add support for using if_pyth to communicate to nREPL server

This change is basically a port of the inline Ruby code that communicates to the nREPL server to Python.

If if_ruby exists, then this change effectively does nothing. If not, then it checks to see if if_pyth is available, and if so, overrides the function that starts a ruby command via the system() call.

My rationale for doing this:
* I want to use Windows.
* I want to use a 64-bit version of Vim.
* There is no 64-bit version of Ruby on Windows.
* Calling an external process seems slow.
  * Also couldn't get this to work, it appears to just hang for me.
  * I am using the latest version of Vim from the mercurial repo. Maybe this is related to how shell escaping has been changed in Window's cmd.exe?
* I want to use Visual Studio 2012 to compile Vim, but I think Ruby doesn't compile under this unless MinGW is used. So, compiling Vim with Ruby bindings is difficult.