New classes and types for handling asset amounts:
  * Quality opaque type for order book quality
  * Amount replacement for STAmount
  * Amounts, in/out amount pair for offers
  * 'core' namespace with type aliases for Ripple primitives.
New classes for processing offers in order books:
* BookTip provides consume-and-step offer traversal
* OfferStream applies offer business rules and presents offers to callers
* Taker class manages state for the active party during order processing
* Offer class wraps book offers for presentation
Improvements to scons build for beast.

* Common code extracted to Python directories.
* Read ~/.scons file for scons environment defaults.
* Override scons settings with shell environment variables.
* New "tags" for debug, nodebug, optimize, nooptimize builds.
* Universal platform detection.
* Default value of environment variables set through prefix dictionaries.
* Check for correct Boost value and fail otherwise.
* Extract git describe --tags into a preprocesor variable, -DTIP_BRANCH
* More colors - blue for unchanged defaults, green for changed defaults, red for error.
* Contain unit tests for non-obvious stuff.
* Check to see that boost libraries have been built.
* Right now, we accept both .dylib and .a versions but it'd be easy to enforce .a only.
Refactor ripple_overlay
  * Use rvalue move to receive accepted sockets
  * Split asio dependent APIs to their own class and file
  * Update documentation
[FOLD] Move predicates to a separate file
[FOLD] Rename to Peer::ptr, remove Peer::ref
[FOLD] Tidy up headers for predicates
[FOLD] Rename to Message
[FOLD] Reshuffle includes