Extension to work with timestamps and time ranges

[v0dro]

It would be useful to have an extension for managing time stamps and ranges at C level, to perform things faster and avoid bloat by using the ruby Time class.

Maybe something on the lines of this: http://docs.scipy.org/doc/numpy/reference/arrays.datetime.html

[translunar]

We did talk about having matrices that could be keyed by strings instead of integers. I think the basic idea would just be to use Ruby hashes to convert whatever datatype to consecutive integers.