Add support for client that uses GET as transport mechanism (#186)

Current implementation always uses POST as the transport mechanism. Adding GET support enables usage of GET queries for caching simply via URL.

Some notes:
- I left the existing API for creating a new client as is, but the implementation could be much cleaner by introducing some sort of configuration struct when creating a new client
- The construction of the query parameters follows the logic from Apollo's client implementation, which can be found here https://github.com/apollographql/apollo-client/blob/8beb4820edc6352996e08f7f73bde3573f1eb666/src/link/http/rewriteURIForGET.ts
- Updated integration tests to use both sets of clients. Updating the tests to use a test suite would be cleaner

internal/integration/schema.graphql

@@ -12,6 +12,10 @@ type Query {
   fail: Boolean
 }
 
+type Mutation {
+  createUser(input: NewUser!): User!
+}
+
 type User implements Being & Lucky {
   id: ID!
   name: String!
@@ -21,6 +25,10 @@ type User implements Being & Lucky {
   friends: [User!]!
 }
 
+input NewUser {
+    name: String!
+}
+
 type Hair { color: String }   # silly name to confuse the name-generator
 
 type Animal implements Being {