Added a couple of tests to check possibility of creation of custom converter object.
Renamed ArgtParserException to OptionParserException
Renamed method getSingleValue in Option class to getValue.

Option now throw IllegalStateException on attempt to receive their values when they were not attached to any parser or OptionParser.parse method was called.

All generic options expect no arguments by default now.
nargs(n) means now that option must take exactly n arguments, not from one up to n of them as before.