Bulk create promisify

I've changed the code in bulk create from emitters to promises. Overall the code is shorter, and in my opinion easier to read.

However, I've had to add even more boilerplate code taken from bluebird, in order to return a sequelize promise instead of a bluebird promise from the utility methods (`method`, `settle`, `all` and `try`). 

Comments are very welcome, this should be a good pointer as to where promises can take us :). Going forward, there should be less need to add boilerplate each time we change some code to promises