Smarter Tokens fetching (#3546)

* Don't auto-subscribe for contracts #3240

* Smarter Balance : don't re-instantiate contracts, fetch tokens #3240

* Smarter Balance Tokens fetching (update image when needed) #3240

* Attaching to TokenReg Events instead of fetching for each block #3240

* Unsubscribe from shapeshit... #3240

* Unsubscribe from EthFilter if used once (resolved) #3240

* Remove warning

* PR review fix

* Typo

* Better contract JS API : one subscribe for all #3546

* Fixed test