Add first validators to models

Add validators to the `BuildDate`, `CSize`, `ISize`, `Name` and
`Version` models to ensure positive integers, correct names and
Add the comparator instance methods `Version.is_older_than()` and
`Version.is_newer_than()` which rely on pyalpm's `vercmp()` to compare
two version strings and return whether they are older or newer
Update the documentation for `ISize` to reflect, that it represents size
of an installed package.

Add tests for added validators and comparator methods.
4 jobs for issues/6 in 6 minutes and 20 seconds (queued for 3 seconds)