Ability to add existing contracts

[SwaroopH]

Even though write calls won't work, the ability to fetch data and capture events can be very helpful for developers.

[anomit]

1. Write calls may not work if they are based on some sort of ownership control, like Ownable.

2. Pull request Verify contracts feature #15 adds this feature for contracts that you have the complete source code for.

3. Coming soon the ability to add contracts by their ABI as well as choose from a list of pre-selected contracts.