Create base requirements for tox

tox dependencies can be established in a base environment and
subsequently included in other environments. While most of the
requirements themselves are handled in a separate file, using this
approach will ease potential future changes.
Use better versioning for tox

tox supports specifying a version range. By doing so, Simon's tests can
always be performed against the most up-to-date version of any minor
version of PyMongo (e.g., 2.4.2). As originally constructed, all tests
were performed against the original release (e.g., 2.4).
Add PyMongo 2.5 to tox
Change tox environment names

I think this is more readable.