Clarify boost dependency

[rljacobson]

If you are having trouble compiling-- try getting rid of everything under ### Testing and Benchmarking ### in CMakeLists.txt that way you don't have to download the large boost library (boost isn't clearly listed as a requirement)

Originally posted by @sjlevy in #6 (comment)

Figure out what we're using boost for. Either replace it with a lighter alternative, remove the dependency altogether, or, if it's absolutely necessary, document the dependency and its consequences.