Merge branch 'PixxxeL-box2d-example' into dev

merge #1018; replace tabs with 4 spaces in examples

Replace `==` on `===` in examples

Replace `tabs` char on 4 `spaces` chars in examples

removed texture check from render loop ...

saved on if statement! better than none :)

Made RemoveChild no longer returns error

Update WebGLRenderer.js

Fixed pivot issue

Fixed Canvas RenderTexture ...

Now uses matrix also

Little blendMode tweak

Created new initContext function for webGLRenderer ...

less code = good :)

Removed update and destroy webGL functions

Removed PIXI.texturesToUpdate ...

No longer required

Removed the updateFrames global Array

Updated the Matrix class ...

New functions and a bit of a tidy up

Merge pull request #964 from krzysztof-o/dev ...

fixes #619: correct size of trimmed sprites

Crop and trim as optional parameters

Rewrote update transform ...

matrix now used correctly.

fix typo in event target

Merge branch 'powerfear-doc-fixes-dev' into dev

merge in some doc fixes (#967)

merge bubbling events (#914)

Update CanvasRenderer.js

Merge pull request #946 from DuskEagle/dev ...

Fix WebGlContextLost handling.

Merge pull request #962 from krzysztof-o/patch-1 ...

Graphics.arc anticlockwise is boolean (not number)

Merge pull request #966 from krzysztof-o/get_set_child_index ...

added getChildIndex and setChildIndex methods to DisplayObjectContainer

Merge pull request #973 from krzysztof-o/bitmap_text_kerning_fix ...

fixes #972

Update CanvasRenderer.js

Reduce some logic redundancy in clearRect/fillRect code for Canvas...? ...

Topic.

Add clearBeforeRender for <webgl> ...

Maybe "clearBeforeRender" this is not the best naming (since it will clear anyway..), but this aligns canvas and webgl:

Basically it allows the user to do clearColor whenever they want, instead of this being done for them. This is useful in the following circumstance:

1) The game/program loads a background image, but not right away.
2) Before the background image is loaded, we want to fill the canvas with a certain color (for example, white).
3) In canvas mode, this is done by setting ``clearBeforeRender`` to false and then adding a single ``fillRect`` command.
4) In webGL mode, the equivalent is running ``gl.clearColor`` and then ``gl.clear``.
5) We can of course set the background color in webGL mode and this probably won't be expensive (although I don't know), but in canvas it *is* expensive to clear/fill on every frame.
6) Thus, the user would have to detect webGL mode or canvas mode themselves, and either set the background color or not.
7) To eliminate this coding complexity for the user, this small bit of extra code aligns ``clearBeforeRender`` (or, as much as possible) to the canvas implementation.