Fast Multivariate Normal Densities as .RMD

Submission based on SO thread:
http://stackoverflow.com/questions/17617618/dmvnorm-mvn-density-rcpparmadillo-implementation-slower-than-r-package-includi
as commented
changes made according to Dirk's comments

However, I don't see the relationship to multivariate Normal draws.
Sure, both could be put into an actual package, but I don't see
relationship between the codes.
Merge pull request #33 from ReverendBayes/gh-pages

I'll do a few more cleanup edits once merged.