Update build for gcc 4.8.x:
* Use gcc 4.8.x, boost 1.54
* Honor CC,CXX,PATH environment variables
* Prevent compilation with unsupported toolchains
* Print g++ version during Travis build
* Run built-in unit tests after Travis build
[REMOVE] Allow gcc 4.7 and later
Remove files inadvertently added
PathRequests object to own all path requests.
SHAMapSync cleanups and performance improvements
Order book subscribe cleanups.
Add LES::dirIsEmpty function to check if a directory is empty
Reduce log priorities.
Add io_latency_probe
Limit legacy pathfinding requests
SHAMapSync bugfix. Cannot avoid push in getMissingNodes
Refactor IP::Endpoint
IPAddress is deprecated
PeerFinder work and refactoring:

 * Implement PeerFinder business logic.
 * Support fixed peers (including DNS support).
 * Add journal support to Peer and Peers.
 * Refactor PeerDoor support.
 * Tidy up Peers and eliminate connection functionality and timers.
 * Refactor Peer interface and add journal support.
 * Allow construction of incoming Peer using an existing socket.
 * Remove TESTNET support.
 * Misc. cleanups.