fix tabbing

this PR fixes:
```
pelican /Users/jchen/git/fly/burrito.sh/content -o /Users/jchen/git/fly/burrito.sh/output -s /Users/jchen/git/fly/burrito.sh/publishconf.py
Traceback (most recent call last):
  File "/Users/jchen/.venvs/burrito.sh/bin/pelican", line 11, in <module>
    sys.exit(main())
  File "/Users/jchen/.venvs/burrito.sh/lib/python3.4/site-packages/pelican/__init__.py", line 344, in main
    pelican, settings = get_instance(args)
  File "/Users/jchen/.venvs/burrito.sh/lib/python3.4/site-packages/pelican/__init__.py", line 338, in get_instance
    return cls(settings), settings
  File "/Users/jchen/.venvs/burrito.sh/lib/python3.4/site-packages/pelican/__init__.py", line 56, in __init__
    self.init_plugins()
  File "/Users/jchen/.venvs/burrito.sh/lib/python3.4/site-packages/pelican/__init__.py", line 76, in init_plugins
    str('module'))
  File "/Users/jchen/git/fly/burrito.sh/plugins/thumbnailer/__init__.py", line 1, in <module>
    from .thumbnailer import *
  File "/Users/jchen/git/fly/burrito.sh/plugins/thumbnailer/thumbnailer.py", line 137
    if not filename.startswith('.'):
                                   ^
TabError: inconsistent use of tabs and spaces in indentation
make: *** [publish] Error 1
```