Add support for line-height and variant to the Text component

These work using DOM, but not canvas.  The textFont method now covers all the possible parts of the [font declaration.](https://developer.mozilla.org/en-US/docs/Web/CSS/font).

Fixes #494.
Add get() method to crafty core

This adds a get method that will convert selections to arrays of entities.  It can also be used to fetch a particular specified entity by index.  It will work as expected an a single entity.  This is modeled directly after jquery's get method: http://api.jquery.com/jQuery.get/

Documentation about the new feature is added (it is now referenced by the Crafty Core docs, as well as  .toArray).  Includes a few tests added to core.html.

This fixes #349
Merge pull request #651 from starwed/get-method

Add get() method to crafty core
Merge pull request #652 from starwed/line-height

Add support for line-height and variant to the Text component
Fix example indentation and code style
Merge pull request #653 from potomak/documentation-minor-fix

Fix example indentation and code style
Remove getter/setter ie8 fallbacks
Remove old IE specific filter code from DOM.js
Remove IE6 background hack
Remove documentation note about ie8
Remove IE8 specific rotation code and setup
Remove IE8 test logic
extracted code from extensions.js

moved keys & mouseButtons to keycodes.js
moved zeroFill to 2D.js (only 2D.js uses zeroFill)
fix jump while falling

fixes #419
Merge pull request #657 from starwed/kill-ie8

Kill ie8 specific code in Crafty
Allow visibility MBR to be set manually

This patch adds properites to "2D" that indicate how far beyond an object's default dimensions the visible MBR should be.

These can be set with the method offsetBoundary.
Merge pull request #658 from starwed/manual-mbr

Allow visibility MBR to be manually offset
Merge pull request #660 from mucaho/split_files_1

extracted code from extensions.js
Merge pull request #662 from mucaho/fix_jump_falling

fix jump while falling