Implemented Test:
   - issue two currencies USD and BTC via @currency contract
   - create EXC exchange between USD and BTC
   - buy BTC with USD and then sell proceeds back for BTC
   - verify only error is rounding error in favor of exchange

Added missing Float compilier intrensics
Added flaot64 types to WAVM api
Added print functions for asset in WASM contracts

Fix system account name for EOS token

Use the same as in eosio.system
https://github.com/EOSIO/eos/blob/317606da15730c3c413d9b7ed8507934c14ae320/contracts/eosio.system/eosio.system.cpp#L15

Fix problem when a matching results in a partially filled existing order, the existing order and the bidder/ asker balance are not updated properly.

Added test for basic wasm contract loading/unloading.  Fixed bugs in the intrinsic code and made it impossible to complile with the same class of bugs in the future.  Did some basic style/renaming in the contracts directory and in the enclosed tutorial/doxygen files

Also since the API changes creeped into so much code,
decided to just go ahead and change the remaining code. #624

Fixed many typos

* Fix asks table name
* Handle open_orders per account

- implement a large part of example exchange contract