Tidy up includes
Add ci_char_traits
New http::client_session for making requests:
- Add 10,000 popular URL data set for unit tests
Fix clang/travis, prefer static libs when BOOST_ROOT is set.
Report whether a ledger is validated:
This addresses https://ripplelabs.atlassian.net/browse/RIPD-275
Use Json::StaticString to improve performance.
Use inject_error instead of manually adding errors.
Fix a few warnings
Fix VSProject generator item sort
Autobridging:
Complete implementation of bridged offers crossings. While processing an offer
A:B we consider both the A:B order book and the combined A:XRP and XRP:B books
and pick the better offers. The net result is better liquidity and potentially
better rates.

* Rearchitect core::Taker to perform direct and bridged crossings.
* Compute bridged qualities.
* Implement a new Bridged OfferCreate transactor.
* Factor out common code from the Bridged and Direct OfferCreate transactors.
* Perform flow calculations without losing accuracy.
* Rename all transactors.
* Cleanups.
Improve checking of library versions.

* Correctly handle openSSL version checking on Ubuntu, Debian, OS/X.
* Also check versions of openSSL, Boost, GCC and MSVC at C++ compile time.
* Get rid of over-engineered runtime CheckLibraryVersions.
Revert "Improve checking of library versions."

This reverts commit 69fccdf5c633cf217caeb45e0a8398485d38e194.