Deprecate the default database

[oniony]

On startup, TMSU will create a default database at the default database location ($HOME/.tmsu/default.db) if one does not already exist. The rationale for this was to make it easier for new users to get started by not putting any barriers in their way. However, with the introduction of local databases (created with the init command), the presence of the default database confuses the situation:

• New users may not realise it's even possible to create local databases but instead junk everything into one large database
• The use of relative and absolute paths becomes confusing as the path type is decided by the relationship between the database root path and the path being tagged.

The following changes are proposed:

1. TMSU will no longer create a default database on start-up if one does not exist.
2. If TMSU cannot find a database a message will be printed instructing the user to use the init command.

See issue #49 for backstory.