Depecrate `PIXI.math` namespace ...

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

Remove the rest of internal module coupling between packages

fixed extends doc issue

give a method for plugins to add custom middleware

add .fnt as an xml loader extension

typo in comment

fixes #1709 outdated loader docs

whitespace

expose async and Resource

Remove pixi-spine dependency. Tested on real project :)

less ducktyping, use is* from loader; fixes #1628

Merge remote-tracking branch 'origin/dev' into dev

use pixi-spine plugin

move premade loader to main export

fix some bugs in the loader

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

fix loading of bitmap font

get relative bitmap font loading properly.

better detection of bitmap font; should be relative to xml url now

loader already passes baseUrl, don't need to do it in add

export a premade loader

More docs again !

Docs fixes, improved the consistency of the deprecation, thanks @englercj ...

Also added a comment on the bitmap font parser, will leave the change
until we find a new caching system

fixed typo in cacheAsBitmap, fixed bitmap text and restored the font cache so we can debug it

Merge branch 'alvin/dev' into dev ...

Conflicts:
	src/interaction/interactiveTarget.js

Docs, spine and tiling sprite fixes, another deprecation warning

fix logic error in bitmapFontParser check:

massive jsdoc fixes

fixed typo in loaders/index.js; gulp build wasn't working because of this

some code linting.. ...

nice and tidy!