Fix bug with DSLMixin

[lfdebrux]

Creating a class that inherits from DSLMixin should allow it to use DSL methods, however instead I get a KeyError

Steps to reproduce bug

1. Create a class that inherits from DSLMixin
2. Instantiate the class
3. Call a DSL method

>>> from capybara.dsl import DSLMixin

>>> class MyClass(DSLMixin):
...     pass
>>> mypage = MyClass()
>>> mypage.assert_selector('link', 'Help')

Expected behaviour

The DSL method returns as if calling capybara.dsl.<method>

>>> mypage.assert_selector('link', 'Help')
True

Actual behaviour

A KeyError is raised

>>> mypage.assert_selector('link', 'Help')
*** KeyError: <MyClass object at 0x7f16a350d780>

Suggested fix

DSLMixin should result in methods being attached to the class, rather than functions, so that the DSL methods can be used from instances of classes that inherit from DSLMixin.

I've attached a fix below that includes tests showing the behaviour I would expect.