Change variables regarding connection to database constant

Driver, protocol, and the database file name will not change throughout the
execution of the program. Thus, it is safer to make these private constants.
Change visibility of Statement variable

This variable is often used in the child classes, for example, when executing
a query. Therefore, it was considered better to make this protected, allowing
access only from child classes instead of adding a getter method. However,
adding a getter method is a possibility as further development is done.
Fix bug where tables are not recognized as existing even when they do

The cause of this was because hard coded lower-case names were used for creating tables,
but Derby automatically converted the table names to upper-case. Solutions to
this bug were to use upper-case table names and make sure that the table name
is upper-case by converting them intentionally in the constructor.
Add getter method for variable tableName

Considering the possibility of tableName being modifiable in the future,
a better choice was to use getter instead of making tableName a public variable.
Add abstract methods to the parent class

These 4 methods are used to execute the basic four SQL commands using proper
classes. It might be a better choice not to make these abstract and implement the
four methods in the Database class.
Add method that formats a String specefically for INSERT SQL command

Due to the fact that the INSERT command needs the format (columnNames, ...)
VALUES (val, ...) it was considered better to split the formatting part to a
different method instead of clattering the insert method with the formatting.
Add the four abstract methods inherited from Database

This commit implements the methods from the Database class. Only the insert
method is implemented. However, it might be a better choice to move these four
methods to the parent class because it seems that there is nothing
particularly different between the Records and Preferences
class, which are the only classes that the four methods are currently planned
on being implemented.
Add test for checking whether insert method runs or not

Tester class for the insert method that was implemented in the Records class.
Whether or not the values were properly inserted has not been checked yet.
This test was done only to check if it can be ran without exceptions
happening.