Split pop_and_normalize_style into two. With @bjoska.
Refactorings in dsl.rb, and edit_box 'with_text'.

This is a partial cleanup in dsl.rb, passing the app
in a consistent way to the elements, and putting that
boilerplate stuff in one place.

It also adds missing behavior for edit_box, which can
now be called with an intial text as a first parameter.

We also added some specs for DSL methods that were still
untested. edit_box and image.

Shoes on! @bjoska and @arnebrasseur :metal:
Refactored part of the dsl layer and added further specs for the dsl.

New specs using shared_examples added for the edit_box and edit_line
in editable_elements.

The refactored parts of edit_box, edit_line and progress use the new
create method.

We also froze some constants, because freezing constants is the least
one can do.

happy hacking! @bjoska and @arnebrasseur signing off <3
Fix issues with backend loading, and move backend-specific behavior to the front.

With this patch it becomes possible to do

````
SHOES_BACKEND=swt rspec spec/shoes/app_spec.rb
````

To run a single spec against a specific backend.

Also, Shoes::Swt::EditBox and EditLine were setting defaults styles on the dsl
object. This is *wrong*. Now these defaults are in Shoes::Edit{Box,Line} so
they are the same on all backends.
Add a spec for the check method, and use the DSL#create method for it.
Merge pull request #250 from shoes/pairwithme_for_a_better_dsl

DSL improvements and enhancements to testing setup