Add preliminary Structured Overlay docs
Remove recursions from computeLiquidity
Pathfinding Documentation
Rename RippleAsset to Issue and RippleBook to Book.
Convert STAmount to use Issue:

* Split STAmount out of SerializedTypes.h
* New concept of "Issue consistency": when either both or neither of its
  currency and account are XRP.
* Stop checking for consistency of Issue in its constructor.
* Clarification of mIsNative logic in STAmount.
* Usual cleanups.
SHAMap Documentation
Patch Boost to suppress warnings on clang
[KEEP?] Suppress some non-important warnings on Travis builds