Add QString comparison

[MartinDelille]

Since I'm using Qt I would like to make direct QString comparison without converting it to std::string explicitely. I would see two alternative to this:

• Add the possibility to make constraint with custom type (it is already possible to add new constraint)
• Add an optional set of instruction specific to Qt type like QString

[joakimkarlsson]

I'll have to think about how this could be done in a clean way. I'm not to keen on adding Qt-dependencies in the framework, but perhaps there's a way to make it easier to expand.

This really belongs in joakimkarlsson/snowhouse as that is what igloo uses for assertions.

[MartinDelille]

Replaced by joakimkarlsson/snowhouse#5