parse traces from f2fs events
Created by: kdub
Adds trace parsing for the f2fs_write_begin, f2fs_write_end, f2fs_sync_file_enter, and f2fs_sync_file_exit events.
Test: run the 4 new unit tests (nosetests tests/test_filesystem.py) Change-Id: Ic9a77ea4c29c10de0538e4a9d2e6b20d6dc85064