support specify user to run job

[w136712058]

Some times we need specify user to run some job for the reason such as authority problem (such as file I/O authority).I modify it to solve this problem and it work perfectly on my enviroment.

[coveralls]

Coverage decreased (-11.4%) to 78.488% when pulling 907ca02 on w136712058:master into 978a26d on kraiz:master.

[w136712058]

@coveralls

[w136712058]

@coveralls Why the check result always has “django_crontab/crontab.py:18:19: F821 undefined name 'basestring'”？
Even changed nothing it also occur？
What problem here：

string_type = basestring if sys.version_info[0] == 2 else str # flake8: noqa

or here：

if sys.version_info[0] == 2:
string_type = basestring
else:
string_type = str

[yapici]

@w136712058, thank you for this PR. Do you have plans to work on this to address the failed tests?