Initial specs for feature.

Unit tests for private methods load_due_date load_grace_period.
Fix specs for load_due_date and load_grace_period
Add part specific due_date grace_period capability

Now load_due_date and load_grace_period have the below priority:
1)Part specific due date and grace period in autograders config file
2)Assignment specific due date and grace period in autograders config
3)Default values in EdXClient

If part_name is not supplied in load_due_date or load_grace_period,
they revert back to their previous version
travis
More exhaustive tests for run with timeout
Fix unable_to_parse bug as reported by SJoseph.

This bug has another aspect that becomes apparent when we read
the output of the spawned process so slow, that the process
terminates long before we read its whole output. This alone is
not bad, however the condition thread.alive? was used in the while
loop. This led to the premature termination of run_with_timeout.
To make travis update the build
Merge branch 'unable_to_parse_fix' into per_exercise_due_date_and_grace_period
Merge pull request #18 from gspilio/unable_to_parse_fix

Unable to parse fix
Merge pull request #19 from gspilio/per_exercise_due_date_and_grace_period

Per exercise due date and grace period