Handle data_type changes in forthcoming Galaxy release.

Updated commit with fix from @nsoranzo.

The forthcoming release of Galaxy makes breaking change to history contents API - data_type will no longer represent the short format/file extension of the history dataset - it will now represent the Python module and class.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 attempts to use the new file_ext attribute when it can and falls back to the old data_type attribute for older releases that don't include file_ext. This change should make the various methods that consume data_type within bioblend backward compatible but clients should be made aware that show_dataset is going to return different values as of the October release.

For more information on the Galaxy API changes see https://bitbucket.org/galaxy/galaxy-central/commits/9d152ed#chg-lib/galaxy/model/__init__.py.
Fix libraries.get_folders() documentation. Fix afgane/bioblend#91.
 Add deleted attribute to DatasetContainer.