Python bitset checked in (along with unittests)

I have just checked in the python version of bitset (and unittests!) into the boost_python project directory. Now I need to spend a little time making the API for the python version agree with the C++ version. Some discrepancies around init/constructor, nothing major.

I think I have enough functionality to start on benchmarks.

The python bitset uses a list internally to store bits as, can you guess, zero or one. The first crack used the string representation of a bit rather than a numeric which was not a big problem until I noticed that all operations except init and repr needed to convert the value somewhere.

One gotcha to note: the zero-th bit is rightmost, as it would be if you were writing out a number rather than the string layout where the zeroth bit is on the left.