Rice distribution: extend to b=0, add an explicit rvs method.

closes gh-2164 and gh-2742.

As a bonus, rvs get a bit faster. With current master: 
```
In [10]: scipy.__version__
Out[10]: '0.14.0.dev-efbde60'

In [11]: %timeit stats.rice.rvs(b=3.)
100 loops, best of 3: 4.18 ms per loop

In [12]: %timeit stats.rice.rvs(b=3., size=1000)
1 loops, best of 3: 4.13 s per loop
```

with this PR:
```
In [6]: scipy.__version__
Out[6]: '0.14.0.dev-bf7d8d9'

In [7]: %timeit stats.rice.rvs(b=3.)
10000 loops, best of 3: 45.5 µs per loop

In [8]: %timeit stats.rice.rvs(b=3., size=1000)
10000 loops, best of 3: 192 µs per loop
```