vbridge.ts has signature bridge functionality

[akileshtangella]

I tried creating an IBridgeSide interface and having the SignatureBridgeSide and VBridgeSide classes implement it. But doing this ended up being messy. Some concrete reasons include:

1. Some of the shared methods between VBridgeSide and SignatureBridgeSide such as createBridgeSide have different function signatures.
2. SignatureBridgeSide does not have voteProposal methods so I would have to create empty voteProposal methods in order to implement the interface.

So, I stuck with the original SPEC.