Improve the human-readable description of the tesSUCCESS code:

Transactions that return tesSUCCESS have only been accepted and
propagated on the Ripple network and should not be considered
final until they have been included in a validated ledger.
Use asio signal handling in Application (RIPD-140):
* Use signal_set as cross platform way of handling SIGINT
* Remove polling on main thread for shutdown.
* Add extra logging for received signal.
* Clean up exit handling on error in setup routines.
* Reuse isStopped() from Stoppable for status (could be isStopping() instead).
* Ctrl-C should now work for standalone mode as well on Windows.

Also small fixes to Resolver:
* Add Resolver prefix to logging.
* Fix AsyncObject::removeReference() logic.
* Fix work remaining logic.
Add http::chunk_encode:
This transforms a ConstBufferSequence into a new ConstBufferSequence whose
data is encoded according to the Content transfer encoding rules of RFC2616.
The implementation does not copy any memory.
Fix case where slot==nullptr in Overlay:
This changes the Overlay to correctly handle the case when nullptr is
returned by PeerFinder new_inbound_slot on a detected self-connection.
Cleanup ripple::Ledger:
* Convert static member functions to free functions
* Adopt consistent naming convention
* De-inline code
Tidy up the structure of sources in protocol/:
Split out and rename STValidation
Split out and rename STBlob
Split out and rename STAccount
Split out STPathSet
Split STVector256 and move UintTypes to protocol/
Rename to STBase
Rename to STLedgerEntry
Rename to SOTemplate
Rename to STTx
Remove obsolete AgedHistory
Remove types.h and add missing includes
Remove unnecessary includes in app.h
Remove unnecessary includes in app.h
Remove include app.h from app1.cpp
Simplify the Beast fatal error reporting framework:
* Reduce interface to a single function which reports error details
* Remove unused functions
Fix DecayingSample treatment of the window
Fix RPC handlers to use the results of lookupLedger.
Clarify SetAccount logic and clean up existing code
Fix unit_test suite matching with full names
Replace custom exceptions with std::runtime_error