Add Developer Guide To Bitcoin.org

This pull request proposes to add to Bitcoin.org a Developer Guide providing the equivalent of 70 printed pages of introductions to key Bitcoin concepts.

A preview is available at: http://bitcoindev.us.to/en/developer-documentation

The Guide is not complete---there is much more we plan to write over the coming months---but the version in this pull request is internally consistent and reasonably comprehensive. In addition to the Guide, this pull request also includes a [Developer Reference][] with 90 more pages of detailed technical material, such as RPC descriptions, and a [portal page][] linking to the various sections of both the Guide and the Reference.

To keep comments on this pull request focused, we'd appreciate it if suggestions about possible future content or better ways to organize this information were made on our [Content Suggestions Wiki Page][], restricting comments on this pull request to only things which should be fixed or changed before merging.

If you want to help review but don't have time to read over 150 pages of text, we recommend that you review just one of the sections listed below, preferably one that hasn't been well-reviewed yet. We'll add your GitHub @ name to the Being Reviewed By column for any section where you leave a line comment, and add your name to the ACK column if you ACK any section.

| Section Preview (Source Files)         |  Being Reviewed By     | ACKs From |
|----------------------------------------|------------------------|-----------|
| [Block Chain][] ([BC][])               |                        |           |
| [Transactions][] ([T][])               | @TierNolan                       |           |
| [Contracts][] ([C][])                  | @mikehearn                       |           |
| [Wallets][] ([W][])                    |                        |           |
| [Payment Processing][] ([PP][])        |                        |           |
| [Operating Modes][] ([OM][])           |                        |           |
| [P2P Network][] ([P2P][])              |                        |           |
| [Mining][] ([M][])                     | @luke-jr                       |           |
| [Block Chain & Transaction Reference][] ([BC][BCr] & [T][Tr]) | |           |
| [Bitcoin Core RPCs][] ([A-G][], [H-N], [O-T], [U-Z]) |          |           |

If you leave a line comment or general comment for a particular section, we'd appreciate it if you ACK the section once it meets your expectations. We hope this will allow us to build consensus to merge without everyone having to read the entire text.

Many thanks are owed for the two months of work which went into this pull request. An incomplete, alphabetical list is: @cbeams, @harding, @instagibbs, @mikehearn, @saivann, and @tgeller.

TODO:

* [x] Replace P2PH by P2PKH. Fixed in #400 (merged)
* [x] Replace DOS by DoS.  Fixed in #394 (merged)
* [x] GBT can use a persistent TCP socket, without a persistent session. Fixed in #394 (merged)
* [x] More emphasis on key reuse security risks? Fixed in #395 (merged)
* [x] Contracts section improvements suggested by @mikehearn.  Fixed in #394 (merged)
* [x] Remove assertion that pay-to-pubkey (unhashed) is used in all coinbase txes and is more convenient.  Fixed in #394 (merged)

[portal page]: http://bitcoindev.us.to/en/developer-documentation
[Developer Reference]: http://bitcoindev.us.to/en/developer-reference
[Content Suggestions Wiki Page]: https://github.com/bitcoin/bitcoin.org/wiki/Documentation-TODO
[Block Chain]: http://bitcoindev.us.to/en/developer-guide#block-chain
[Transactions]: http://bitcoindev.us.to/en/developer-guide#transactions
[Contracts]: http://bitcoindev.us.to/en/developer-guide#contracts
[Wallets]: http://bitcoindev.us.to/en/developer-guide#wallets
[Payment Processing]: http://bitcoindev.us.to/en/developer-guide#payment-processing
[Operating Modes]: http://bitcoindev.us.to/en/developer-guide#operating-modes
[P2P Network]: http://bitcoindev.us.to/en/developer-guide#p2p-network
[Mining]: http://bitcoindev.us.to/en/developer-guide#mining
[Block Chain & Transaction Reference]: http://bitcoindev.us.to/en/developer-reference#block-chain
[Bitcoin Core RPCs]: http://bitcoindev.us.to/en/developer-reference#remote-procedure-calls-rpcs
[BC]:  ./393/files#diff-57bb2465c82e97a81a1e53e2229df4d9
[T]:   ./393/files#diff-7eee69d80500d9206b05ce690e279c60
[C]:   ./393/files#diff-eeae34c903720dbf4cbe81f171b46b85
[W]:   ./393/files#diff-bed7c96ee2abb3b3777bc917b9011c18
[PP]:  ./393/files#diff-a9003346dc7c6e14a4832e85eda10624
[OM]:  ./393/files#diff-48e1d3c785aa407d1fc85f3198a8dace
[P2P]: ./393/files#diff-2c59de7e497930987874729f4a2f77e3
[M]:   ./393/files#diff-cf7f98cb5aa0669b9fcdc8b38a47a3cd
[BCr]: ./393/files#diff-814453f9c3a6d1f630d2d882b2e6e9c2
[Tr]:  ./393/files#diff-182e681f8c403cdccde68cf8579b468b
[A-G]: ./393/files#diff-61ab80cdacbe0294b1ee1865cd539d42
[H-N]: ./393/files#diff-f39ab1e01bb268901e88fcf2bf900ee8
[O-T]: ./393/files#diff-45ded1c4bc588581662cb3268c4190b5
[U-Z]: ./393/files#diff-fc4a0c8c7a09cc25194744704892b91b