Handle datatype changes in forthcoming Galaxy release.

The forthcoming release of Galaxy makes breaking change to history contents API - data_type will no longer represent the short format/file extension of the history dataset - it will now represent the Python module and class.

This commit modifies getDataType() to work the same either way - but since for the newer version it isn't literally returning 'data_type' attribute - I have deprecated this method. The new method getDataTypeExt() should be used instead.

This commit also adds getDataTypeClass() that always returns this 'data_type' attribute (but the actual method name only makes sense when taretting a newer verion of Galaxy).

For more information on the Galaxy API changes see https://bitbucket.org/galaxy/galaxy-central/commits/9d152ed#chg-lib/galaxy/model/__init__.py.