Added MySQL back-end logic

1) This commit adds MySQL back-end logic through a PHP script which is
hosted on the localhost.

2) Strings.xml has been updated and the layout.xml files for each
activity have been updated to use strings.xml values instead of explicit
strings inline.

3) A new package called DB has been added to encapsulate the Database
handler logic and remove it from the activity files.

4) SQL table information has been added in the form of a MySQL .sql file
exported from PhpMyAdmin.