Missing required option doesn't throw

This is a proposed fix for: https://github.com/airlift/airline/issues/22

This approach introduces a ParseResult object which allows for the collection of multiple parse errors from the user input
and then provides this back to the caller to decide what to do. E.G. send the user the to the help dialog.

This keeps the existing API and fits with the desire to keep the parsing logic agnostic to the details of the HelpOption.

The existing pattern still fails fast with an exception:

```java
MyCommand command = SingleCommand.singleCommand(MyCommand.class).parse(args);
```

This fix introduces the following pattern from the caller's perspective:

```java
ParseResult parseResult = new ParseResult();
MyCommand command = SingleCommand.singleCommand(MyCommand.class).parse(args, parseResult);
command.helpOption.runOrShowHelp(parseResult, command);
```

The call to runOrShowHelp will:
* Run the command if there are no parse errors and help has not been requsted
* Will display help if it is requested regardless of whether there are parse errors
* Will display parse errors and help when there are parse errors
Modified POM for forking

Removed parent pom reference and changed orgId coordinates to allow new fork in maven central
Rev'ed to 0.8 and added Sonatype parent

Sonatype requires it's pom be the parent to support uploads. Followed steps outlined here: https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide
[maven-release-plugin] prepare release airline-0.8
[maven-release-plugin] prepare for next development iteration
Allow for CommandMetadata injection to all subcommands

This is helpful for commands to be able to show the Help text if they have a validation failure
Publish sources & docs on SNAPSHOT builds