Added cppzmq as git subtree

This pull request removes the cppzmq submodule and adds cppzmq as git subtree. This makes the use of the library a lot easier for other projects. It is also more convenient for users to have all in a single git repository.
See http://blogs.atlassian.com/2013/05/alternatives-to-git-submodule-git-subtree/ for details about how to use git subtree.