filters.py: fix "TypeError: format requires a mapping"

Signed-off-by: Wei Liu <liuw@liuw.name>
front.py: import markdown from the right file

Per 0962e34e8 (Move everything into filters) markdown funciton is moved to new
location.

Signed-off-by: Wei Liu <liuw@liuw.name>
tests: import markdown from right file

This should fix travis failure.

Signed-off-by: Wei Liu <liuw@liuw.name>
Merge pull request #65 from liuw/minor-fixes

Minor fixes