Introduce Metamask Subprovider.

MM has a number of inconsistencies with other providers when implementing the JSON RPC interface. This subprovider wraps those nuances so they do not leak into the rest of our code

packages/0x.js/src/index.ts

@@ -53,7 +53,13 @@ export { OrderWatcher, OnOrderStateChangeCallback, OrderWatcherConfig } from '@0
 
 export import Web3ProviderEngine = require('web3-provider-engine');
 
-export { RPCSubprovider, Callback, JSONRPCRequestPayloadWithMethod, ErrorCallback } from '@0xproject/subproviders';
+export {
+    RPCSubprovider,
+    Callback,
+    JSONRPCRequestPayloadWithMethod,
+    ErrorCallback,
+    MetamaskSubprovider,
+} from '@0xproject/subproviders';
 
 export { AbiDecoder } from '@0xproject/utils';
 
@@ -68,7 +74,6 @@ export {
     OrderStateInvalid,
     OrderState,
     AssetProxyId,
-    SignerType,
     ERC20AssetData,
     ERC721AssetData,
     SignatureType,