Allow "label" to be ommited from the field template tag
Support foreign keys in field templatetag
Add template for species field, for use with field template tag
Modify plot detail to use field template tag
Merge pull request #237 from maurizi/topic/speciesField

Editable species field
Fix plot detail view to check for species

The view was crashing when trees did not have species because it was
trying to reference tree.species.itree_code without first checking
tree.species.

Closes #239
Merge pull request #241 from jwalgran/fix/239

Fix plot detail view to check for species