docs(extension): add details about extensions directory

Reverse engineered from Boilerplate: 
https://github.com/aurajs/boilerplate/tree/master/app
#263 Fix IE8 and IE9 logging
Merge pull request #264 from steffenmllr/master

Fix IE8 + IE9 Logging
Make Function.prototype.bind faster and work with ie8
Merge pull request #265 from steffenmllr/master

Make Function.prototype.bind play nice with ie8
Merge pull request #260 from edouard-lopez/patch-1

docs(extension): add details about extensions directory
Ignore .sass-cache
exposes EE2 .once interface, closes #261
Merge pull request #277 from Buritica/261-sandbox-once

exposes EE2 .once interface, closes #261
Factor 'sandbox.on' and 'sandbox.once' with a single 'attachListener' function
Merge branch 'master' of git://github.com/aurajs/aura