Cleaned up, and implemented next_f32 and next_f64.

As of rust-lang/rust@e49be7aae3b33ba9afa1e517022fab3558af1aba ,
`next_f32` and `next_f64` have been added to the `Rng` trait. The two
methods are implemented in Grieverheart/dsfmt-rs to take advantage of
the ability of dSFMT to generate double precision random floating-point
numbers by avoiding the expensive conversion from integer. In light of
this, `genrand_close_open` has been removed.
Fixes #1.