fixed bug in BufferedLogger
adjusted logger tests to use multiple threads logging concurrently
removed some pass-by-references in BufferedLogger
removed some pass-by-references in SimpleLogger
removed unnecessary include
fixed include guards in BufferedLogger
added a rudimentary logging typedef'ed 'interface'
Merge branch 'feature/logging' of github.com:mrks/hyrise into feature/logging

Conflicts:
	src/bin/units_io/BufferedLoggerTests.cpp
	src/bin/units_io/SimpleLoggerTests.cpp
	src/lib/io/BufferedLogger.cpp
	src/lib/io/BufferedLogger.h
	src/lib/io/SimpleLogger.cpp
	src/lib/io/SimpleLogger.h