Include sys/types.h within rc4.h

This makes sure the rc4.h header can be included anywhere,
without assuming that the user of the header needs to include
any special header beforehand.

This fixes compilation on OS X.
Use uint8_t instead of the nonstandard u_char type in the rc4 code
Merge pull request #7 from mstorsjo/rc4-types

Make the rc4 implementation slightly more portable