JNI KernelDensityEstimator

After some profilling, identified that BIOHMM spends >95% of its
running time inside KernelDensityEstimator.estimate(...). Rewrote
as a C++ class, compiled with -O3, hooked back in with JNI, and
now iterations are much quicker. Tested JNI on mac, pushing to
github to get to mellifera and test there.