Assets detection and tokens balances controllers (#19)

* assets controller tokens

* Update PreferencesController no more tokens responsability

* Update AssetsController handling collectibles

* Update tests according to new AssetsController

* Fix some lines on AssetsController

* AssetsController using preferences selected address and assets getter

* AssetsController tokens object with selected address as keys

* AssetsController test tokens per account

* AssetsController tokens by selected address respective test

* AssetsController update new tokens when adding token

* AssetsController add and remove collectibles per account

* AssetsController assets per account and network

* AssetsDetectionController basic structure and web3

* AssetsDetectionControllers methods for token detection and collectibles placeholders

* update AssetsDetectionController test

* web3 changes

* AssetsDetectionController detecting tokens

* AssetsDetectionController handle web3 unset

* AssetsDetectionController correctly detecting tokens

* AssetsDetection collectibles first approach

* AssetsDetectionCOntroller correctly detecting new collectibles, if contract has tokenOfOwnerByIndex method

* AssetsDetection handle ERC721 standard tokens

* Collectibles following standard autodetection

* AssetsDetection fully api based collectibles

* AssetdDetection clean up

* AssetsDetection improve documentation

* AssetsDetection tests WIP

* AssetsDetectionController improve documentation

* AssetsDetection bignumber/web3 addition and full line jest coverage

* AssetDetection hide private methods and add BigNumber type

* AssetdDetection handle web3 send async issue

* new AssetsContractController for interaction with asset contracts

* AssetsDetection full coverage

* fix web3 bignumber dependency

* expose AssetsContractController to API

* detecting new assets only when account changes or polling period

* add fetch-mock where it is possible

* restore script test

* add TokenBalancesController

* expose TokenBalancesController to API

* add collectible image util

* add missing documentation