add exceptions related to sort
move sort to its own module
use SortFactory
set sort to None if no sort order is defined
Merge pull request #321 from aparo/develop

Release 0.20.1
Initial deprecation cleanup
Move general test setup in pyes. Initial test migration out of deprecation
Added migration script. Migrated tests
Fixed some tests. Improved migration script.
No reason to forbid empty values()

Django queryset.values() does not force it and pyes code as well.
add sort tests
Merge branch 'master' of https://github.com/aparo/pyes into sort
pep8, more tests
IdsQuery: check for list instance, not basestring

Ids may be strings, ints, whatever. Better to check if 'values' parameter
is a list, if not, wrap the value into a single-element list.
Fix typo
Merge pull request #326 from freelancer37/patch-1

Fix typo
Added missing import in managers.py.
Merge pull request #327 from abelldh/fix-mapping-import

Added missing import in managers.py.
fix DateHistogramFacet reference from 'zone' to 'time_zone' and add additional params
Merge pull request #329 from stevencdavis/master

fix DateHistogramFacet reference from 'zone' to 'time_zone' and add params