Remove installed mplexporter directory before installing if necessary
Removed rm and rsync with shutil.rmtree and shutil.copytree,
Error checking and moved shutil methods into seperate subroutine for clarity
fixed bug to support python 3
added check for existing submodule source directory
Merge pull request #180 from jakevdp/clean-mplexporter-install

Remove installed mplexporter directory before installing if necessary