Lots of changes have been made during the past weeks.
This PR aims to bring the last changes required for the v2.2 release!

This new version of the library brings the following changes:

• Port of the library on Windows thanks to the great work of Mike Moening. Thanks again to him!
• Support of acknowledgement callback for redis_subscriber::subscribe and redis_subscriber::psubscribe (see wiki)
• Addition of a flexible logging system (see wiki)
• io_service is no longer a singleton and can be set globally or specifically for each redis client (see wiki)
• More compilation customization are available (see wiki)
• More tests have been provided to test the behavior of redis_client and redis_subscriber (integration tests) and provide more stability in the future.
• Travis has been setup
• A wiki is now available, providing more complete and clearer documentation.
• Several bugs or typo reported during the last months have been patched.

Therefore, the library is now portable and is more stable and flexible than ever.

What's next?

• more tests
• performance improvements
• multithreaded io_service support (already supported on windows, not on unix)
• timeout support
• any interesting suggestion!