message: Add fetchMember to MockGuild
diff: @@ -17,6 +17,13 @@ export class MockGuild {
  });
  }
  + public fetchMember(id: string): Promise<MockMember|Error>{
  + if (this.members.has(id)) {
  + return Promise.resolve(this.members.get(id));
  + }
  + return Promise.reject("Member not in this guild");
  + }
  +
  public _mockAddMember(member: MockMember) {
  this.members.set(member.id, member);
  }
  