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.
Update WebGLRenderer.js
removed old comment form Graphics

complex primitives yeehaaaww
doc type in movieclip
fix #890
Fix for blendMode bug in CanvasRenderer

The stage background (fill) was affected by the last used blendMode. In my case the stage quickly turned white after having PIXI.blendModes.ADD Sprites being rendered last on my stage.
Merge pull request #1045 from SebastianNette/patch-2

Fix for blendMode bug in CanvasRenderer
Lots of jsdocs and spelling fixes.
test update
Merge branch 'dev' of https://github.com/GoodBoyDigital/pixi.js into dev
Merge pull request #1046 from photonstorm/dev

Lots of jsdocs and spelling fixes.
updated tests
jsdoc fixes for all of the Filter classes.
Merge pull request #1047 from photonstorm/dev

jsdoc fixes for all of the Filter classes.
jsdoc fixes for Geom classes.
Merge pull request #1048 from photonstorm/dev

jsdoc fixes for Geom classes.
jsdoc fixes for all the Loader classes. SpriteSheetLoader is redundant and should be removed from the grunt tasks imho.
Merge pull request #1049 from photonstorm/dev

jsdoc fixes for all the Loader classes. SpriteSheetLoader is redundant a...
jsdocs fixes in Graphics (are the consts going to move into the geom classes? pretty please :)
jsdoc fixes for the Text classes.