Better ordering of style rules and properties

[Hikariii]

This PR solves the same issues as #159 and #155, and more.
It assigns all issues reported in #140.

This pr too takes priority into account by sorting the rules via the already available method, but sorting happens later (when you need it) when rules are inserted, not when they are parsed and collected.

When processing properties it does not just overwrite the properties but it unsets and adds them.
This ensures that the order is correct and the css like the one below is processed correctly too:

td  {
    padding: 0;
}

td  {
    padding-bottom: 20px;
}

td  {
    padding: 0;
}

The specificity tests are updated to reflect the changes made with ordering the rules and properties.
It also tests all the different uses cases mentioned in #140.

[stof]

@tijsverkoyen please also tag a new release with all the recently merged patches

[tijsverkoyen]

Done, just released 2.2.0