Use a copy of the `p` argument in `Categorical` constructor.
Merge pull request #74 from JuliaStats/db/categorical

Use a copy of the `p` argument in `Categorical` constructor.  This allows Ints to be used for the argument.  Also made the code somewhat terser
Added alias table and Huffman coding tree for sampling discrete vars. Alias table has best performance, so replaces drawtable in Categorical & Multinomial.