Add file field to LanguageTree

Configure media storage in settings.template
Configure load.py to save tree files
Tree loading stores files and looks up by iso code
API view for getting newick string from tree

For a LanguageTree with ID 5:

GET /api/v1/newick_tree/5
Merge pull request #140 from NESCent/tree-storage

Attach trees files to LanguageTree, API endpoint for getting newick strings