Provide enum values for argument usage help

Previously users could have a useful argument name or useful argument
values:

    @Argument(required = true, usage = "usage")
    Unable to parse command line arguments: Argument "" is required
     [PENNY | NICKEL | DIME | QUARTER] : usage

    @Argument(metaVar = "coin", required = true, usage = "usage")
    Unable to parse command line arguments: Argument "coin" is required
     coin : usage

Now arguments with metaVar yield:

    @Argument(metaVar = "coin", required = true, usage = "usage")
    Unable to parse command line arguments: Argument "coin" is required
     [PENNY | NICKEL | DIME | QUARTER] : usage

Which more closely matches the option behavior:

    @Option(name = "--coin", required = true, usage = "usage")
    Unable to parse command line arguments: Option "--coin" is required
     --coin [PENNY | NICKEL | DIME | QUARTER] : usage
Merge pull request #76 from andrewgaul/argument-enum-values

Provide enum values for argument usage help