updateinfo: *_date attributes as Python datetime objects

[bochecha]

This makes the UpdateRecord.issued_date and UpdateRecord.modified_date be actual Python datetime objects, rather than plain strings.

A few caveats, on which I'd like feedback/pointers:

1. Internally, the structure still stores strings. That's because I didn't really want to modify the C API, only the Python one. So the getters and setters for these 2 attributes are doing the conversion.
2. The Python header inclusion (#include <python2.7/datetime.h>) is obviously wrong. Having never used cmake before, though, I can't seem to figure out how to include the proper header, based on the version of Python detected at build time. 😕
3. In order to use Python datetime objects in a C extension, it is necessary to use the PyDateTime_IMPORT macro first. Currently, I'm calling it in the constructor of the UpdateRecord class, which is not necessarily ideal. Where would be the proper place to call it?

Other than that, it makes for a much nicer API:

>>> from datetime import datetime
>>> import createrepo_c as cr
>>> 
>>> up = cr.UpdateRecord()
>>> up.issued_date = datetime.now()
>>> 
>>> up.issued_date
datetime.datetime(2014, 8, 8, 18, 33, 32)

[Tojaj]

Hi, merged, thank you!

ad 1) I am thinking about changing the C API as well, but for now let's keep the logic in the bindings.
ad 2) and 3) see 4339f97