refactor: abstract checkbox presentation to standalone component

[crisbeto]

Moves the presentation logic from the md-checkbox to a md-pseudo-checkbox that can be re-used in other components, like md-select and md-selection-list. Renames and refactors a bunch of CSS classes, unit tests, properties etc.

Relates to #2412.

[crisbeto]

Sorry for the huge PR @jelbourn, but there's no new functionality, just moving stuff around. The main thing is the MdSelectionModule. It's intended to contain the functionality from #2562 as well.

[crisbeto]

Closing in favor of #2619

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