Merge branch 'dev' ...

# Conflicts:
#	bin/pixi.js
#	bin/pixi.js.map
#	bin/pixi.min.js
#	bin/pixi.min.js.map

Fixed a typo in the Mesh documentation.

fixed initial mesh values

Added Plane to Mesh class ...

handy for making simple mesh’s
custom shaders now work with mesh shader too

Massive documentation update; fixes many issues. ...

Issues fixed by these documentation updates:

- #1973, fixed filter and BitmapText property docs
- #1908, fixed type reference links
- #1856, fixed name of MeshShader

Mesh interaction

Depecrate `PIXI.math` namespace ...

Access math members from PIXI when outside of core, require the math module directly when inside of core.

optimize texture property access to bypass the getter

fix rendering problems with mesh and rope

fixed extends doc issue

Rope supports textures from spritheet

fix const usages in plugins

Move items into their namespace ...

This commit also:

 - fixes some dangling whitespace,
 - fixes some incorrect docs in extras/mesh,
 - moves Text to core
 - moves BitmapText to extras
 - moves math constants to const.js

render methods are private

minor doc cleanup

Moved the mesh folder to the root