added "file/tags/add" to api.py

There is a need to work a bit on that function but I would like to start discussion about it.

I think it makes sense to have a helper function for find files and attach tag(s) to multiple files.

Later we could add tags to the following find criteria: 
for entry in ['md5', 'sha256', 'ssdeep', 'tag', 'name', 'all']:

Example:
curl -F sha256=aa620f1588d0a1f937fb593375b31a39e112bdfefc7a08faa2d4fc133f053fe2 -F tags='foo bar' -X POST 127.0.0.1:8080/file/tags/add