fix dates parse

When I run the `selfstats -P chrome --clock 00:00 --tactive` I got error:

```
Traceback (most recent call last):
  File "/usr/local/bin/selfstats", line 9, in <module>
    load_entry_point('selfspy==0.3.0', 'console_scripts', 'selfstats')()
  File "build/bdist.macosx-10.9-x86_64/egg/selfspy/stats.py", line 590, in main
  File "build/bdist.macosx-10.9-x86_64/egg/selfspy/stats.py", line 205, in do
  File "build/bdist.macosx-10.9-x86_64/egg/selfspy/stats.py", line 367, in calc_summary
  File "build/bdist.macosx-10.9-x86_64/egg/selfspy/stats.py", line 289, in filter_keys
  File "build/bdist.macosx-10.9-x86_64/egg/selfspy/stats.py", line 265, in filter_prop
  File "build/bdist.macosx-10.9-x86_64/egg/selfspy/stats.py", line 104, in make_time_string
NameError: global name 'who' is not defined
```

or run `selfstats --date 2014 12 23 --limit 1 d -P vim --tkeys` I got:

```
Max three arguments to date 2014 12 23
```

This pull request fix these bugs.