Finish proposals and gateways feature

[a1300]

Dear @liangpeili, please assign this issue to me!

TODO's copyied from #21

TODO

1. Proposal.js Line 72
   1.1 Variable keys is not used. Variable can be removed
2. For the propsal.upvote function, it would be easier to pass the transactionId as a variable, instead in the options object
   2.1 same for proposal.activate
3. In proposal.js please rename
   3.1 activate -> activateProposal 3.2 upvote->upvoteProposal 3.3 Maybe you could also provide the functionrevokeProposal`
4. For initgateway and I would make the name optional. Instead of hardcoding xxxxxxxxxx. You could make this parmaeter optional.
5. Add proposal.revokeProposal contract function
6. Add Typescript definitions for proposal.js and gateway.js