addRelations feature

Closes #1671 and #1742 

All association methods now return promises, which made the code a bit shorter. To be able to return promises I had to also change model.update and destroy to return promises in order to be able to proxy the SQL. 

The adder is in plural, I think that makes the most sense @mickhansen - It might give issues with associations that end in s, but thats a general issue with pluralization that should be solved elsewhere in my opinion. The issue also affects hasOne / hasSingle, which will also be overwritten, so this should really be fixed by finding a new pluralization lib soon.

I've made the addSingle method proxy to addMultiple if it recieves an array

I've also changed the name of the bluebird package to make tests pass again :)