SQLite step wrapper instead of print, header file

New type step_t, which is a typedef'd sqlite3_stmt, to hide inner
workings of the API from the outside. This is now returned from
list_tags and filter_tag. The results from the query is aquired with the
new step_result function.

The header file ftag.h is introduced to show the public API.

Extra: whitespace errors are removed.
Made dbconn static
Added extern to header function declarations
Removed TODO comments
New function: free_step

The step_t type can, and should now be freed using this new function.
The sqlite3_stmt in filter_tag and list_tags are now non-static, to make
it possible to be stepping more than one stmt at a time.