BeforeAll / AfterAll / -ErrorAction Ignore

Knocking out some low-hanging fruit with this one.  I've updated the code to take advantage of -ErrorAction Ignore if the user is running PowerShell v3 or later, keeping it from polluting the global `$error` variable unnecessarily.

I've also added BeforeAll and AfterAll commands, which work much like BeforeEach and AfterEach, except the code only executes once at the beginning and end of its parent Context or Describe block.  It's syntactic sugar for this:

```posh
Describe 'Something' {
    try
    {
        # <BeforeAll Code>
        # <Describe Body>
    }
    finally
    {
        # <AfterAll Code>
    }
}
```