More robustly validate signature inputs, closes #313
Don't create peer doors in standalone mode
Autobridging hooks and cleanups:

- Detect when we should autobridge and get bridge books.
- Move applyOffer from STAmount into the OfferCreate transactor.
Must set flags before signing validation.
Add standalone testsuite builds to SConstruct
[WIP] Autobridge test implementation
[WIP] Autobridging
[WIP] Autobridging