Restructure dirs

Changes include:

* thread_pool is now in the namespace 'concurrency'  and is under the subdirectory of the same name
* Moved http into a subdirectory, including version.hpp and constants library
* Moved the mime stuff into a subdirectory (this is already out-of-date though)

Next steps:

* Build the examples again
* Clone gtest and gmock from their SVN repositories and put the projects in cpp-netlib organization
* Add submodules for gtest under the project cpp-netlib/deps
* Add my updated URI project as a project in cpp-netlib organization, and add it as a submodule in the cpp-netlib project
* Eventually concurrency, mime and logging could be submodules

With the submodule structure and gtest/gmock dependencies in place, then it should be OK now to start refactoring the HTTP lib according to the design doc you wrote several months ago.