Feature/cli fixes

[abridgett]

Corrected a duplicated help text.
Also pulled a workaround from http://stackoverflow.com/questions/23349349/argparse-with-required-subparser so that "airflow" prints help rather than an ugly help with python3 (Ubuntu 14.04):

Was:

[2016-01-05 22:31:02,400] {driver.py:120} INFO - Generating grammar tables from /usr/lib/python3.4/lib2to3/Grammar.txt
[2016-01-05 22:31:02,429] {driver.py:120} INFO - Generating grammar tables from /usr/lib/python3.4/lib2to3/PatternGrammar.txt
[2016-01-05 22:31:02,553] {__init__.py:36} INFO - Using executor CeleryExecutor
Traceback (most recent call last):
  File "/opt/airflow/bin/airflow", line 15, in <module>
    args.func(args)
AttributeError: 'Namespace' object has no attribute 'func'

Now

2016-01-05 22:14:43,790] {driver.py:120} INFO - Generating grammar tables from /usr/lib/python3.4/lib2to3/Grammar.txt
[2016-01-05 22:14:43,820] {driver.py:120} INFO - Generating grammar tables from /usr/lib/python3.4/lib2to3/PatternGrammar.txt
[2016-01-05 22:14:43,943] {__init__.py:36} INFO - Using executor CeleryExecutor
usage: airflow [-h]
               {backfill,clear,trigger_dag,run,test,task_state,webserver,scheduler,initdb,resetdb,upgradedb,list_dags,list_tasks,worker,serve_logs,flower,version,kerberos}
               ...
airflow: error: the following arguments are required: subcommand

[mistercrunch]

nit: spaces around operator pls

[abridgett]

fixed - sorry, really should go and get pylint etc all hooked in properly

[mistercrunch]

I use flake8 with the git commit hook
https://pypi.python.org/pypi/flake8

It will give warning only for the things you've touched which is nice. Other linters sometimes warn or alter more than what you bargain for.

Thanks for the PR!