Added generic_text attribute to SimpleEntity

A simple TEXT field was added to `entity.py` which can store any kind of textual data such as JSON strings (using e.g. `json.dumps( jsondata )`).

I was able to verify that `test_db.py` ran smoothly and worked fine yesterday. But when I merged the develop branch, just recently, re-applied my new code and ran test_db.py I'm getting errors on everything. I have not digged deeper into this but I suspect this can have something to do with me running a MySQL db again...

`test_simple_entity.py` runs all tests without errors.