Added BeforeAll and AfterAll commands

Per community request.  BeforeAll and AfterAll is syntactic sugar for:

Describe 'Describe Name' {
    try
    {
        <BeforeAll Code>
        <Describe Body>
    }
    finally
    {
        <AfterAll Code>
    }
}

Both command can also be used inside a Context block with the same effect.
Clean $error variable

Code now uses -ErrorAction Ignore instead of -ErrorAction SilentlyContinue when the user is running PowerShell v3 or later.

Because of the various scopes in play during Mocking and InModuleScope, the code to look up whether to use Ignore or SilentlyContinue had to be exported from the Pester module in some way.  I opted for a function instead of a global variable, as it's less fragile.
Removed commented code
Merge pull request #237 from dlwyatt/Before_After_All

BeforeAll / AfterAll / -ErrorAction Ignore