Mixer and Mixin Recipes proposal

See https://github.com/kristianmandrup/rfcs/blob/master/active/0001-template.md

Makes it easy in Ember to mixin various orthogonal functionality (much like Aspects) into places where the functionality should be "attached". Makes the core classes adhere more to Single Responsibility.
A PostRoute should not be concerned with what security strategy it uses or if it logs debugging output etc. This should be handled as an auxiliary concern.

This proposal defines: 

- A basic efficient mixin API, by default based on regular expression matching on names.
- A Recipe declarative language, for defining mixin recipes as an object structure that is parsed, and the mixin API is then called to reflect the declarations. 

For now I think this should live in its own add-in project and not be part of core until it is a proven (accepted) idea/concept. But let me know what you think of the general idea.