Fboemer/cast cleanup

Add some cleanup missed in !81 (merged):

• More new => make_shared (closes #86 (closed))
• Use static_{pointer_}cast when dynamic_{pointer_}cast type is assumed to be correct
• Remove more relative include paths (closes #81 (closed))
• include/gtest/gtest.h => gtest/gest.h
• Some cleanup of header include orders