Properly order warning suppression flags during compile
Fix build warnings and .gitignore.

* Comment out unused local function for both clang and g++.
* Get rid of numerous Boost warnings for clang.
* Remove some unused local variables.
* Put TAGS into the .gitignore.
Make bind_handler variadic ctor explicit
Add HTTPHeaders::build_map
Enable asynchronous handling of HTTP-RPC (RIPD-390)
* Activate async code path
* Tidy up HTTP server code
* Use shared_ptr in HTTP server
* Remove check for unspecified IP
* Remove hairtrigger
* Fix missing HTTP authorization check
* Fix multisocket flags in RPC-HTTP server
* Fix authorization failure when no credentials required
* Addresses RIPD-159, RIPD-161, RIPD-390
Documentation for Ledger Consensus implementation (RIPD-405)
Fix auth handling during OfferCreate (RIPD-414)
Fix intrinsic calls in static_initializer
Tidy up hardened_hash:
* Added siphash as a HashAlgorithm
* Refactored class responsibilities
Tidy up hardened containers (RIPD-380):
* Rename hardened containers for clarity
* Fixes https://ripplelabs.atlassian.net/browse/RIPD-380