Adds random number generation for BigInts from GMP
Refactor BigInt RNG
Add a default BigRNG and the standard interface to seed BigRNGs
Add BigFloat random number generation
Improved support for random BigInt ranges

This means that ranges between numbers bigger than `typemax(Int)` work
now, but range lengths are still limited to up to `typemax(Int)`.
Add normally-distributed BigFloats generation
srand() now seeds both RNGs
Use `shasum` instead of `sha1sum` in OS X

OS X doesn't have `sha1sum` available, so the fallback used to get
the initial seed whenever /dev/urandom isn't available doesn't work.
Add documentation to the BigRNG
Merge branch 'bigrng' of github.com:andrioni/julia into andrioni-bigrng

Conflicts:
	base/gmp.jl
	base/mpfr.jl
	doc/stdlib/base.rst
Merge branch 'master' of github.com:JuliaLang/julia