Adding outline for local database management

'DatabaseContract' class defines table name, fields, and create/delete
methods for use in further database manipulations.

'DatabaseHelper' class manages the creation and update of the database.
Adding scene table to database setup

Hooked in the scene table, which references the character table.
Modified the message table to reference the scene table instead.
Adding 'choice_selected' field to choice table
Removing uniqueness constraints on database

A temporary measure, until it becomes clear which uniqueness
constraints should be enforced (if any).
Re-organising strings for use in SQL queries
Adding JUnit test class for database

Currently only tests creation of database. Must include I/O
tests, to ensure that database functions as expected.
Completing basic JUnit testing on database

After the creation of tables, mock data is inserted
and retrieved. The retrieved query values are then
compared with the original mock data to ensure that
values and data type are preserved.

Additionally, scene, message and choice records cannot
be inserted without an existing record in their respective
dependent tables, so referential integrity is also tested
at a basic level.

A slight change is also made to the choice table schema
to reduce a redundancy in timestamping. Only the date of
choice selection must be recorded by the client. This field
is created with a null value initially.
Merge branch 'database' into develop