Add a "Common Problems" page to the documentation

[pipermerriam]

What was wrong?

There are certain issues that are encountered regularly by users.

How can it be fixed?

We should add a "Common Problems" page to the documentation which tries to capture these succinctly and provides answers/solutions.

• using web3.eth.sendTransaction when connected to infura
• trying to access web3.personal API when using parity or when the personal API is not enabled with geth.
• trying to call methods off of the Web3 class rather than an instance of the class.
• improper instantiation of contract instances like trying to call functions off of the contract class.

[wolovim]

Finally closing this ancient issue after making a couple doc tweaks. I like the pattern of adding an FAQ to the bottom of specific guides, e.g., contracts, otherwise questions that don't fit nicely into one bucket wind up in Troubleshooting.