Intro update

[yiannisbot]

This PR applies a major update to the intro section of the spec.

• It introduces some basic concepts, which are central to understanding the Filecoin network and protocol. Although these concepts are also described later on in the specification, it is important that the reader is aware of them from the beginning.
• It also introduces a new subsection on the lifecycle of data in the filecoin network, key to understanding the workflow.
• It would help to have a similar section on the lifecycle of transactions and blocks.
• It removes the diagrams (for now) as they seem out of date.
• It does not remove the VM description, as I believe it still adds some clarity, despite having its own section later on.
• It also adds descriptive text on the Filecoin Economy, giving a high-level view of the storage and retrieval markets.
• It introduces the concept of verified clients. The same description is also included in the CryptoEconomics section, but given it's an important concept, I don't mind the redundancy.

@nicola: it will probably take several iterations to bring this to a final final stage, but I believe this is a good start on which I would like to work together to revise.