Refactor & introduce new tagged types:
* Categorize *all* non-generic instances of uint160
* Introduce new types ripple::Account, ripple::Currency,
  ripple::Directory and ripple::NodeID
* Extract to_string functions from header to hide dependencies
* Move conversions to and from strings to central .h file..
* New IssuedCurrency struct.
* Get rid of constructors and comparators between tagged uint types.
* New base_uint::copyFrom() allows explicit copying between any uint types
  of the same size.
* Include what you use and C++11 for cleanups.
* Extract macro constants to be functions.
* Lots of incidental cleaning and removal of unused code.

Respond to comments
SHAMap refactoring:
* Rename SHAMapNode to SHAMapNodeID.
* Cleanups
Ledger loading cleanups:
* Fix close time settings for ledger load
* More info in ledger_request and inbound ledger JSON replies