Set Underline Style of Text Block

Addresses issue #237 

Sets the underline style if it is nil, "none", "single", "double", and "error"

Unfortunately, "low" is not a style option based on the documentation if you look at options when setting the underlineStyle field:
http://help.eclipse.org/indigo/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/swt/graphics/TextStyle.html#underlineStyle