Allow {% thumbnail %} 'size' to be an alias name. See #248.
 - could extend to allow variables, not just hardcoded  strings as aliases
 - not sure what needs to be done to take alias 'target' into consideration
Merge remote-tracking branch 'DrMeers/thumbnail_tag_aliases'
Alter some of DrMeers alias code
Remove a redundant step of Aliases.get
Update CHANGES and bump number for release