Implemented ceil and log2 builtins.
Implemented ceil and log2 builtins.

surround-io/oclgrind,bitfusionio/Oclgrind,bitfusionio/Oclgrind,surround-io/oclgrind,bitfusionio/Oclgrind,bitfusionio/Oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind,surround-io/oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind,surround-io/oclgrind