Fix bug in list_cards without a limit

I had forgotten to catch TypeErrors. Oops.
Merge pull request #4 from zachallaun/fix-list-cards

Fix bug in list_cards without a limit
nicer limit arg parsing
API_KEY never expires
correct behavior of limit=0
fix formatting bug and add limit arg
better completion behavior (not a fix to issue #5)
add verbose option that as yet does nothing
display board it when being set
attempt to fix packaging
make card limit optional
add readme.rst for pypi
Rewrite of main library for clarity
more flexible cached_accessor decorator
add 'show' action to show card or board in webbrowser
bash completion with show command
readme feature suggestion
Keeps empty lines in messages

This is important for things like code formatting, where a newline
between comment text and code-block text is significant.