Work-in-progress support for retrieving PG_DIAG result error fields.
Expose the Diagnostics object in the extensions module
Added all supported properties to the Diagnostic object
Added test to verify Diagnostics reference disposal
Added test to verify Diagnostics works after copy errors
Decode Diagnostics result on Python 3
Added documentation about the Diagnostics object
Diagnostics is more GC friendly
Diagnostics added to the NEWS file
Fixed bad interaction of setup.py with other dependencies

The problem is in Distribute dependencies on Python 3. Create a new command
class instead of changing inplace the one other projects may use.

Close ticket #153.
Merge branch 'diagnostics' into devel