Merge pull request #31 from FreekDB/master

Fixed some Checkstyle issues (after running "mvn checkstyle:checkstyle" and checking "target\checkstyle-result.xml")
Addressed TODO in DataEntryForm.java: 
TODO: can we use JOptionPane.showConfirmDialog(null, message, "title",
JOptionPane.YES_NO_OPTION)? [Freek]
displayNoReportsFoundDialogue(...) method now uses
JOptionPane.showConfirmDialog(...).

Removed redundant constants. 
Added useful constants.
Addressed TODO in DataEntryForm.java: 
TODO: can we use JDialog instead of JFrame (that way we can set the
owner to the viewer frame)? [Freek]

DataEntryForm now extends JDialog. 
Added new constructor DataEntryForm(ViewerFrame parent).
ViewerFrame.java now uses above constructor of DataEntryForm. 

Minor changes in Main.java.
DataEntryForm:
- Fixed some small Checkstyle issues (after running "mvn checkstyle:checkstyle" and checking "target\checkstyle-result.xml").