Prefixed repository classes with "Package"

Because that's what they really are.
pep8
Disable logging, add docstring
A warning for myself
pep8
I can't see any reason for InstalledPackage to be instanciated without meta data
Added is_package_like
Moved package spec parsing code to the utils module
Make package repository objects iterable
Added a FormulaRepository class
pep8
Check for mandatory formula attributes
Added NameVersionRevisionComparable mixin

It use old code used to compare packages.
Now both package and formula classes inherits from this mixin, helping
in coherent comparisons between instances.
Missing import
Ignore python coverage directory
Missing imports
Tests for mixins and a fix for a version and revision comparison bug found while writing the tests
Fix a bug in revision comparison
Return the package file path
Added utils.make_package_spec