Add-Type command

Implemented some parts of the [Add-Type command](http://technet.microsoft.com/library/hh849914.aspx).

The following are now supported:

```
Add-Type -AssemblyName 'Microsoft.Build, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

$source = 'public class MyType { }'
Add-Type -TypeDefinition $source

$method = 'public string WriteLine() { return "Test"; }'
Add-Type -MemberDefinition $method -Name "AddMemberDefinitionUsingDefaultNamespaceTestClass"

Add-Type -Path MyAssembly.dll

$type = Add-Type -PassThru -MemberDefinition $method -Name 'MyClassName -Namespace 'MyClassNamespace'
```

Limitations:

Only supports C#.
No support for generating an assembly from source code.
Do not see compiler errors written to the error stream only the terminating error (unlike Microsoft's PowerShell). They are available from the $error object.
No wildcard support.