Adding namespace prefixes to python class typeNames.

This was formerly impossible due to limitations in IECore.registerRunTimeTyped, but will be possible if this pull request is accepted :

https://github.com/ImageEngine/cortex/pull/57
Renamed WriteNode->ObjectWriter and ReadNode->ObjectReader.

Fixes #17.
Merge pull request #451 from johnhaddon/readNodeRename

Read/Write node renames