ADD: Tcp connector

DONE
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Socket connectors
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 * Create SocketClient and SocketServer connectors that can be used for JSON-RPC via TCP socket (not really sure UDP is sufficient but can be activated).
* Server can be configured to handle multiple connections at the same time. By default, it only handle one connection at a time.
* Compilation of Socket connectors is conditioned with SOCKET_CONNECTOR CMake variable.

Fix
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I have corrected the way connectors headers are installed

Improvement
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Library dependency uppon pthread is integrated into CMake configuration using Threads package

TODO
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Refactor to use Windows thread instead of pthread on Windows platforms
To complete Pull Request #50, we should remove this line.
In deed connection is created outside of the server and should not be deleted here.