Fix Issue 658, max ksize silliness in the C++ code

This is a fix for https://github.com/ged-lab/khmer/issues/658.

I factored out the maximum ksize, using the definition from the hash functions. I amended the conditionals in the python interface to the hash functions to test against the MAX_KSIZE constant, and changed the string for the exception thrown when the user supplies a larger ksize so that the exception string refers to the MAX_KSIZE constant.