Implement C++14 std::make_reverse_iterator
Refactor base_uint<> and STAmount::getText:

base_uint<>:
* Remove unnecessary typedefs and derived classes
* Factor out common code and eliminate redundancies
* Add tag support to improve type safety
* base_uint::ToString() -> to_string(base_uint const&)
* base_uint::GetHex() -> to_string(base_uint const&)

STAmount::getText:
* Don't overallocate
* Eliminate unnecessary copying
Use last closed ledger for old pathfinding
Rework handling of modified ledger nodes:
* Track dirty nodes in a set
* Make flushDirty a member function
* Don't write deleted nodes
* Make sure new nodes are shareable
* Put new nodes in the treeNodeCache
Keep SHAMapNodes that fault in handling client requests.
Refactor RippleCalc.cpp:

* Add comments
* Restrict code to 80 colums
* Remove boost::format
* Remove BOOST_FOREACH
* Make members private
* Add ripple_unordered_set to UnorderedContainers.h
* Replace boost::unordered_set with ripple::unordered_set