Rename to RCall and join JuliaStats

[ViralBShah]

This came up in an earlier discussion: JuliaLang/METADATA.jl#1617

Given that this package is quickly becoming the standard way to call R, it may be worthwhile to follow the same naming convention as ccall, PyCall, JavaCall, etc.

Just a thought - Perhaps the package can also become part of the JuliaStats github organization that may make it easier for others to contribute.

[lgautier]

Yes. JuliaStats is where I think this should move. I am aiming at closing this for the end of the year.

[lgautier]

Contingent on issue #43