Fix RIPD-166:

* Allow RPC commands sign and submit to sign using the regular key.
* Allow command line RPC command submit to take an "offline" flag.
* Mark SerializedObject.getFlags() as const.
Universal flags need to be added to tfAccountSetMask
Tidy up some source filenames:
* Add .unity suffix to mark unity sources
* Fix some relative include paths
Add VSProject SCons Builder
[WIP] Improved rippled SConstruct