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

LanguageTree model now has a `file` field
load/tree.py now attaches trees files to LanguageTree models when creating them
An API endpoint has been created to serve newick strings from a LanguageTree object.

`GET /api/v1/newick_tree/<language_tree_id>` returns a json object like `{'newickTree': '...'}`

See #112 and #108