Added the "topics" table.
app: implemented the TopicsCreate function.
app/lib: created the app/lib directory.

This directory will hold functions and structures that are handy for this
particular application. Moreover, I've moved the Options struct into its own
file data.go, where it's now named ViewData.
app: moved models into their own files in the new subpackage "models".

The goal here is to make these models reasonably fat in order to keep
"controllers" thin.
Added more methods to the models.

This way "controllers" become thiner.
app: created the subpackage config.
app: removed the config directory and moved some code here and there.
Moved the routing into its own file.
Moved the lib package outside the app subpackage.

It makes sense since the lib files are meant to be independent from any "app"
source.
Fixed the tests for the routing.
app/models: added tests for all the models.
Ported the main package to the new models.Exists function.
app: using the models.CreateUser function instead of a copycat.
app: the DB truncation doesn't seem right.