Start updating tx21 description
more updates to tx21, more needed
The bulk of tx21 description updates
Tx21 description updated, plus other cleanup

Version and doc history to be updated.
clarified sort order for tx21 applying existing orders
added blocktime to the sort for matching tx21 orders
clarified tx21 unit price calculation, and other clean-up

Please give this a close read to make sure the whole tx21 description is correct.
Intermediate update after group review

still needs:
* .3% liquidity bonus
* rounding up for fractional quantities of indivisible coins
intermediate clarification of tx21
another interim save
clarified liquidity fee, rounding for tx21
added liquidity bonus examples, fixed liquidity typo

removed satoshi reference
Updated 0.4.5.10 release date
Manual merge branch 'marv-engine-tx21'
fixed conflicts before pushing tx21 to my repo
Merge branch 'master' of https://github.com/mastercoin-MSC/spec into HEAD

Conflicts:
	README.md
Merge pull request #165 from marv-engine/tx21

Tx21 (Sell one currency for another MSC-based currency) description updated. Removed conflicts w/ help from @CurtisLacy.