integrate external exchange through adapter

[gabririgo]

since our exchange will not go in production, it is important to integrate oasisdex ERC20 exchange exchange asap

logic: implement through adapter (library vs assembly logic)

[wnz99]

Picking up from previous discussions and https://github.com/RigoBlock/contracts/issues/16

It might be possible to sign transaction offchain on behalf of a fund and send to 0x:

https://0xproject.com/wiki#Intro-Tutorial:-Create,-Validate,-Fill-Order

[gabririgo]

using libraries won't do the job, we have to keep using assembly. Good as not much changes in the code are needed. Preparing for testing oasisDex integration this week