Add pending txs to tx list

[danfinlay]

A few notes:

• Signed messages are not currently persisted, so after they're approved, they leave the list. I believe this was a conscious design decision we made, but I can't find the rationale. I think we might want to add persisting of signed messages for review.
• I've been hitting some out of gas errors, but I don't think it's associated with this branch.
• When first refreshing the plugin, when a site requests web3.eth.sign, it first reloads the page as if we switched providers. This only happens on the first sign request.

@kumavis