Multi-Threading Support, Reliability Improvements, Better Non-Rails Projects Support

Most of the major changes in this version are related to our initial push towards making
DbCharmer thread-safe and making sure it works correctly in multi-threaded environments.

Please note, that even though we now test DbCharmer in multi-threaded environments, we still
consider multi-threaded support experimental.

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Bug fix: Improved Rails environment detection (sometimes DbCharmer would use
Rails-specific code while running in non-rails projects).

Bug fix: Make sure on_db() method could restore original connection after an exception is
raised from a DB driver during connection switching (special thanks to Dmytro Shteflyuk for finding
the issue and helping with debugging).

Important change: This is the first release that does not have a really strict constraint for point-releases
within the Rails 3.2.x branch.