Default to release build type

Set CMAKE_BUILD_TYPE to release. This adds the 03 flag to the compilation of c/c++. Factor 2x improvement in speed! (for the random function I chose).

timeit results on test_uranium_cascade pyne:staging 
10000 loops, best of 3: 121 µs per loop
timeit results on test_uranium_cascade crbates:build_type
10000 loops, best of 3: 64.7 µs per loop