Enforce the correctness of Package objects

[pombredanne]

The Package object holds quite a bit of data, and it is pretty loose right now in checking what is expected.
You can construct an incorrect one pretty easily.
We need some validation of sorts, possibly by defining some schema/validation library.
There are many such libraries so the hard part is picking a sensible and simple one.

[pombredanne]

Of these, http://awesome-python.com/#data-validation this one http://schematics.readthedocs.org/en/latest/ seems the most promising at first glance.

[balusarakesh]

planning to use the above mentioned validator http://schematics.readthedocs.org/en/latest/.

[balusarakesh]

When Model class (schematics package) is inherited into Package class it doesn't let us create new variables and throws an exception like

raise ModelConversionError(errors, data)
          ModelConversionError: {'release': 'Rogue field', 'epoch': 'Rogue field', 'arch': 'Rogue field'}