implement bold, italics and underline

bold and underline are not yet supported when they are combined
implement background and foreground color
update Readme.md
move colorWithHexColorString into a category
implement NVMTextView

this moves code from the appdelegate handling the contents of the textview
into its own NSTextview subclass.
implement NVMClientWindowController

moves code from app delegate handling the client window into its own
class