Sqlitedict

* Read-only mode (nrhine1, 37)
* Check file exists before deleting (adibo, 39)
* AttributeError after SqliteDict is closed (guyskk, 40)
* Python 3.5 support (jtatum, 47)
* Pickle when updating with 2-tuples seq (Digenis, 49)
* Fix exit errors: TypeError("'NoneType' object is not callable",) (janrygl, 45)

* fix regression where iterating over keys/values/items returned a full list instead of iterator

* improve error handling in multithreading (PR 28); 100% test coverage.

* full python 3 support, continuous testing via Travis CI.