Implement += assignment.

Implementation uses dynamic to add two objects together.

It looks like PowerShell is not using dynamic. Trying to add two System.Objects together
from the PowerShell command line results in the error:

Method invocation failed because [System.Object] doesn't contain a method named 'op_Addition'.

So it looks like PowerShell is using reflection for some types and probably handles types
such as integers, which have no op_Addition method, individually by casting them to the
correct type.
Merge pull request #158 from mrward/assignment-by-addition-operator

Implement += assignment.