Const in decode/open memory.

"const" added in stb_vorbis_decode_memory and stb_vorbis_open_memory to
allow to read from const buffers.

I'm storing the raw data from the ogg files in const buffers. When I try to decode from memory, VS2010 gives me some errors related to the const stuff. This change fix these errors.

![pull_request](https://cloud.githubusercontent.com/assets/1184254/4069053/d5e0a80c-2e3f-11e4-8da8-802f672f04b6.png)