Parallelization and RNG

[coatless]

Parallelization

To speed up the bootstraps, we can parallelize different portions of the code. In order to do that, we need to enable stored seeds. This is a more concrete fix of #80.

My thoughts:

1. Launch a sitmo R package with linkto headers. [x] see: coatless/sitmo
2. Add seed = to generating functions.
3. Remove calls to R::r* in favor of std::gen* or use boost (ziggurat will be faster for normal)