Add 'signTransaction' method to DappBrowser

The DappBrowser still does not handle 'Sign Transaction only'.

*I think* to handle this we will need to add an additional shim in the init.js provider which is identical to the 'processTransaction' hook, only called 'processSignTransaction'. This hooks into the sign transaction code in the AlphaWallet Provider (alphawallet.js). I have added all the backend methods to handle signing only in DappBrowserFragment and DappBrowserViewModel.

Investigate how to do this - you will probably need to write a dapp that has the signTransaction method as I couldn't find any. Suggestion: use the code here: https://github.com/JamesSmartCell/JamesSmartCell.github.io and add the sign Transaction. You'd need to check the signature of the RLP code you receive, so you'll need to use Ethers.js for that - I think I already included it.

Once this is in, we can re-deploy and use it as a handy testbed.