Add setProvider method to 0x.js that updates the web3 provider and invalidates any contractInstances instantiated with the old provider

src/0x.js.ts

@@ -133,6 +133,14 @@ export class ZeroEx {
         this.web3Wrapper = new Web3Wrapper(web3);
         this.exchange = new ExchangeWrapper(this.web3Wrapper);
     }
+    /**
+     * Sets a new provider for the web3 instance used by 0x.js internally and invalidates any instantiated
+     * contract instances created with the old provider.
+     */
+    public setProvider(provider: Web3.Provider) {
+        this.web3Wrapper.setProvider(provider);
+        this.exchange.invalidateExchangeContract();
+    }
     /**
      * Signs an orderHash and returns it's elliptic curve signature
      * This method currently supports TestRPC, Geth and Parity above and below V1.6.6

src/web3_wrapper.ts

@@ -9,6 +9,9 @@ export class Web3Wrapper {
         this.web3 = new Web3();
         this.web3.setProvider(web3.currentProvider);
     }
+    public setProvider(provider: Web3.Provider) {
+        this.web3.setProvider(provider);
+    }
     public isAddress(address: string): boolean {
         return this.web3.isAddress(address);
     }