Improvements to SConstruct:
* Default target is release instead of debug (scons with no arguments).
* All targets now include debug symbols, including release.
Rationale: "out of the box" builds of rippled using plain "scons" or "scons -j4" will produce
a debug instead of a release build, which could underperform.
Build gcc.debug using -Og flag

Since gcc 4.8 is required anyways, it might be nice to use its features.

Intro to feature (second bullet point):
https://gcc.gnu.org/gcc-4.8/changes.html

-g (line 328) is still needed:
http://stackoverflow.com/questions/12970596/gcc-4-8-does-og-imply-g
Add +DEBUG to the raw version string for DEBUG builds.
This will show up in the rpc server_info command.
There is no impact on the version string for release builds.
Mark several Ledger member functions as const.
Decouple LedgerMaster from configuration
Distinguish Byzantine failure from tx bug (RIPD-523)
Return correct error message for invalid fields
Add --quorum command line argument (RIPD-563)
Make path filtering smarter (RIPD-561)
* Break path liquidity checking into its own function
* Measure initial quality over minimum destination amount
* Test for available liquidity
SHAMap performance improvements (RIPD-434)
 * Remove mTNByID
 * Remove mDirtyNodes
 * Much faster traverses
 * Faster snapshots
 * New algorithm for flushing
 * New vistNodes/visitLeaves
 * Avoid I/O if a map is unbacked
[FOLD] Suggestions based on code review
[FOLD] Missing canonicalize calls