change build system to OMake

Only ca3a4cdc9b2a289e851ed2fd00c689498cdf8c0b is necessary. This causes build failure. I went further and manually packed the `Riakc` module. Also, I switched to using OMake. I wouldn't mind if you rejected these. However, I do think the new build is cleaner. There are fewer files and the source directories are not polluted (everything goes under _build).