Added functions for connecting to DB

Added some basic functions for connecting to and disconnecting from the DB using the
database config file.

Signed-off-by: Sebastian Lenartowicz <slenarto@uoguelph.ca>
Added uniqueness constraint to username

Made the username field in the "Users" table unique.

Signed-off-by: Sebastian Lenartowicz <slenarto@uoguelph.ca>