Make Graphical Wizard into own package

[leeper]

The graphical wizard is nice, but it would be helpful to be able to develop it separately from the main package. That would also remove the tcltk dependency here, thereby allowing for easier installs and slimmed down package NAMESPACE.

TODO:

• Remove graphical wizard code
• Remove graphical wizard documentation; mention graphical wizard code in documentation
• Add link to wizard to MTurkR README
• Remove wizard from NAMESPACE exports
• Remove tcltk and grDevices::bringToTop from imports

[leeper]

Repo created here: https://github.com/leeper/MTurkRGUI
Initial commit made: cloudyr/MTurkRGUI@f1f19a4