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The forthcoming release of Galaxy makes breaking change to history contents API - data_type will no longer represent the short format/file extension of the history dataset - it will now represent the Python module and class.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 for objects library dataset downloads.

My best guess is there was a subtle bug in this code - because it was passing library dataset ids to an API endpoint that was expecting LDDA ids. LDDA ids would match LD ids in all the tests I guess because an LD never has its LDDA replaced I don't think.

Regardless - the API endpoint has changed so that now it does infact expect an LD id - but the parameter name has changed as well so I have fixed this here. More information on the Galaxy change can be found here - https://bitbucket.org/galaxy/galaxy-central/commits/c0c00d69cae3966a5c1e67abc3ba7aa67a735a64.

But this is definitely breaking - the new object library client will not work on older versions of Galaxy anymore and the older version of the client will not work with Galaxy after the October release. @simleo @nsoranzo is this acceptable - should we revisit the Galaxy API and make it more backward compatible to still allow LDDA IDs (I don't think it is worth it because of the bug mentioned above - but maybe I am wrong about the bug or maybe it is common to never replace a dataset so LD ids always match LDDA ids). Pinging @martenson on this as well.