add crc32 function

This function is about 26x faster than the native julia implementation I wrote
in the CRC32 package (https://github.com/fhs/CRC32.jl). I wish I knew zlib had
crc32 before, but it doesn't hurt to have both implementations available in
case we don't want to depend on libz.

Best timing out of 5 runs:

```
julia> data = rand(Uint8, 10^8);

julia> @time CRC32.crc32(data)
elapsed time: 4.949734025 seconds (1599999776 bytes allocated)
0x9b716e0b

julia> @time Zlib.crc32(data)
elapsed time: 0.189449411 seconds (64 bytes allocated)
0x9b716e0b
```
Merge pull request #6 from fhs/crc32

add crc32 function