Fixed IBM xlC compiler error due to missing prefix.
Add atomicops for AIX/POWER + IBM xlC compiler.
Added contributor and updated README to add AIX.
Modify tests to compile on AIX with xlC compiler.

- `int64` typedef collision with `sys/inttypes.h`
- `xlC` complains about `int` and pointer comparisons without casts
Merge pull request #69 from apaprocki/aix-atomicops

Add AIX/POWER atomicops and fix compilation with IBM xlC C++ compiler.