Refactored the bump::Log to be fully thread-safe which completes Issue #4.

Previously, only the std::ostream of the Log was actually thread-safe. The Log system needed to be modified so the entire system was thread-safe. This commit addresses the issue. The convenience functions now only accept a single bump::String argument instead of using the << operators.
Added the smallsha1 library and the CryptographicHash class information to the README doc completing Issue #5.
Incremented the version number to 1.1.3-15.