Various cleanups:
* Replace SYSTEM_NAME and other macros with C++ constructs
* Remove RIPPLE_ARRAYSIZE and use std::extent or ranged for loops
* Remove old-style, unused offer crossing unit test
* Make STAmount::saFromRate free and remove default argument
Add ZeroCopyOutputStream and tidy up
Split peer connect logic to another class (RIPD-711):
All of the logic for establishing an outbound peer connection including
the initial HTTP handshake exchange is moved into a separate class. This
allows PeerImp to have a strong invariant: All PeerImp objects that exist
represent active peer connections that have already gone through the
handshake process.
Use ppa:afrank/boost 1.57 for Travis
Combine STTx::checkSign overloads:

Callers don't need to specify the signing key -- they're just retrieving
the key from the SerializedTransaction and then passing it back.

This simplifies Ed25519 implementation.
Add appveyor