HTTP Client Test Consolidation, Remove Boost.Test

This is the simplification of the HTTP client test suite which serves as
a guide for getting other tests migrated to use gtest. Moving forward
the idea is to accomplish the same for all of the tests in cpp-netlib:

* Consolidate logically grouped tests into a single binary.
* Remove all traces of dependency to the Boost.Test framework.
* Get all tests to a "green" state again.

This paves the way for implementing mocks for some internal classes that
we want to be able to change. In the meantime simplifying the test suite
buys us more in the long run as we add more and more tests to the
library.
Migrate message, request, and response tests
Migrate logging tests to gtest.
Merge pull request #11 from imaginaryboy/gtest-integration

Migrate logging tests to gtest.
Migrating concurrency tests to gtest
Run clang-format on the http directory.
Run clang-format on message/
Run clang-format on concurrency/
Fix clang-format booboo on macros.
Run clang-format on contrib/
Run clang-format on logging/
Merge pull request #197 from deanberris/gtest-migration

Migrate all tests to gtest
Updated uri submodule.
Merge remote-tracking branch 'deanberris/clang-format-everything'
Adding quotes for CXX_FLAGS, to prevent building failure
Add cppnetlib-logging for http test executable
Merge pull request #211 from w0land/master-integration

Master integration
Fix issues #110 and #168.
Merge pull request #216 from skystrife/master

Fix issues #110 and #168.
Add ability to compile with clang without libc++.

To do so, supply -DCPP-NETLIB_DISABLE_LIBCXX=ON to cmake.