Remove --output option
Add output LEMONdB as a positional argument
Update usage message
Remove code that updates the input LEMONdB
Improve error message if input LEMONdB does not exist
Remove Bash completion for --output
Force the user to give the output LEMONdB as a positional argument

The TypeError ("coercing to Unicode: need string or buffer, NoneType
found") exception is raised when the input LEMONdB is updated with the
computed periods. Rather than just fixing this, use the same approach
as in 'photometry' and require the user to specify the output LEMONdB,
therefore removing the possibility of updating the input one. Thanks
to Scott Engle for letting us know about this problem (#20).

This merges branch 'periods-add-output-db-as-positional-argument'
Add Scott Engle