add test for a wrong logfile
add test for parsing travis log
add test parsing stages with incorrect tags
check build started and finished tags for a complete stages
test empty started and finished
add test for started and finished timestamps with incomplete stages
check return value of parse_job_log_file
add print wrapper, that only prints message when verbosity level matches
print debug messages depending on verbosity level
add add_stage() to build class
use Build instance in favor of Stages instance
parse travis worker tag from log file
rename travis timing tags template constant