[WIP] Workflow build step

Changes to allow the plugin be used in a Workflow.  In particular... look for `TODO` markers in the changes I made.  These were things I was not sure about.

Atm it works using the base 'step' step (i.e. you need to specify the $class) e.g.
```
node('master') {
  // Do stuff....
  step($class: 'hudson.plugins.copyartifact.CopyArtifact', projectName: 'project1', filter: 'hellothere.txt')
  // More stuff
}
```

Once the workflow apis are published (?) we can build a dedicated step for this, it would look something like the following:
```
node('master') {
  // Do stuff....
  copy(projectName: 'project1', filter: 'hellothere.txt')
  // More stuff
}
```

I refactored the `perform` parts of the `CopyArtifact` class a little.  I did not change the basic logic, just composed it a bit better (I think) in an effort to make sure I understood how it worked.  We can keep that refactoring if we think it's an improvement in the readability of the code.  Otherwise... I'll back it out and minimize those changes.