Add usage property to flag.

Added the usage property to the flag type, also handle default
values for usage.

Affected issues:
	surtrstudios/weyland#5
commandline::Parser::RegisterFlag returns the flag.

Affects issue:
	surtrstudios/weyland#5
Add usage property to Parser.

This returns the usage for the flags registered to the parser.

Affected issues:
	surtrstudios/weyland#5
Write help message when command line error occurs.

This closes surtrstudios/weyland#5
Fix broken tests for Parser.

Resolves build breakage in:
https://travis-ci.org/smkell/weyland/builds/23699364
Merge pull request #16 from smkell/development

Add support for writing help message when command line errors occur