Tx51 (crowdsale) version 1 accepts bitcoins

We need to specify the block number where tx51 version 1 will be turned on.

* tx51 version 1 allows 0 (bitcoin) for Currency Identifier desired
* early bird bonus no longer tied to weeks
* number of tokens issued limited by max Number of Coins value
* Number of Coins now a signed 8-byte integer
Reverted early bird bonus percentage to original description
added multiple currency support to tx51; need answers to 

https://github.com/mastercoin-MSC/spec/pull/185#issuecomment-45281671
https://github.com/mastercoin-MSC/spec/pull/185#issuecomment-45284533
Clarified which tx51 fields can be changed
clarified Participating in a Crowdsale desc.

tweaked the wording to account for a crowdsale accepting multiple currencies.
replaced all "tx" with "Transaction type" in the text
a couple of tweaks from today's group review
Merge pull request #185 from marv-engine/tx51_accepts_btc

Tx51 (crowdsale) version 1 accepts multiple currencies, including bitcoins