RF: use catch_warnings for checkwarn module

Simplify code, by not checking if new filters have been loaded on top.

The code was also causing errors in python 3.3 because we weren't
restoring warnings errors correctly.
TST: add python 3.3 travis build
RF: use dcm_data get for dicomwrappers get

Confusingly, we were using 'getattr' on the underlying dcm_data, for
dicomwrappers 'get', and for `__getitem__`.  Use 'get' on dcm_data
instead.  Add tests.

This has the happy feature that you can now use 'get' and '__getitem__'
with tags, to get stuff from the dcm_data attribute.
RF: require dcm_data argument to wrappers

It was a bit confusing that we defaulted to an empty dictionary when
instantiating dicom wrapper objects without any input arguments.  Make
clear that the wrappers do need a dicom-info-providing thing.
Merge pull request #173 from matthew-brett/better-dicomwrapper-get

Better dicomwrapper get

DICOM sprint - simplify the interface to dicomwrappers by making wrapper 'get' defer to self.dcm_data.get, and similarly for __getitem__.

Also - make the dcm_data a required input argument to the wrapper to be clear that it is required for sensible functioning of the object.
Merge pull request #172 from matthew-brett/py33-fixes

RF: use catch_warnings for checkwarn module

Simplify code, by not checking if new filters have been loaded on top.

The code was also causing errors in python 3.3 because we weren't
restoring warnings errors correctly.
merge