Added a rest method to copy an item.  Added the abaility to copy a file to the file model.  Adjusted the gridFS and S3 assetstores so that duplicated files don't need to duplicate data.
Added tests for copying items and for some other cases that were not covered.  Fixed an issue in testing for blank metadata keys.  When downloading a multi-file item, I've added a sort parameter so that the files are in a predicatable order.
Fixed a python style complaint.
Added test coverage for S3 deletes.
Don't copy the file _id when duplicating the file.
Move the item copy API from POST item/{} to POST item/{}/copy.  Changed some variables for greater style consistency.  Added some more tests to ensure copying is really doing the expected job.
Moved the copyItem functionality into the item model.
Modified some of the tests of the S3 assetstore to use moto.  Explicitly add the appropriate file key so that we can test that it is deleted.
The travis build was failing.  @cjh1 suggested that I try asking travis to uninstall six.
Merge remote-tracking branch 'upstream/master' into item_copy

Conflicts:
	.travis.yml
	requirements-dev.txt
Added a comment to .travis.yml to explain why we are upgrading six.
Merge pull request #337 from manthey/item_copy

Item copy.