Add An Abstract StreamWriter

For one of my projects, I already created a Class handling streams instead of SplObjectInfo

refs #86

Also add A utility test class StreamWriterTest for easier test stream
creation.

Signed-off-by: Benoît Burnichon <bburnichon@gmail.com>
I agree, it could be usefull. 

But at the moment, we don't use it, so i would remove it.
It leads to dead code