v0.2 propsal - abstract/interfaces only implementation + concept docs

A v0.2 proposal featuring:

* implementation changed to abstract/interfaces only
* removed all previously implemented tests
* improved and re-organized the concept documentation

Any feedback, comments and/or changes are welcome as always :) 
I'll release to npm once merged, so that existing core implementations can be upgraded. 

Further improvements/changes to the concept and its abstract implementation can be made upon demand as before ;)