Cleaner namespace & doc using roxygen2

[miraKlein]

• Replaced Depends with Imports in the description, because loading all four packages when loading ggbiplot is not necessary.
• All the documentation (also data) is now created via roxygen2.
• Created a (mostly) proper namespace with @importFrom statements instead of library().

[trickytank]

Loading the other packages confused me for a bit. I think I might just start using your fork @miraKlein