ENH: New function regressProjections.R

New function regressProjections.R is a convenient method for performing
regression on eigenanatomy projections from image sets.  Documentation
(regressProjections.Rd) is also included.
WIP: Fixed regressProjections to take eigenvectors.
ENH: Updated regressProjections.Rd with reproducible example.
Fixed documentation for regressProjections.
ENH: Removed vegan dependency.
ENH: Enabled use of glm options in regressProjections.