Pandas Datetime Fixes and Dropping Python 3.5 Support

This release removes support for Python 3.5.

Modified pandas datetime/timedelta handling to be represented as a string with proper format strings in swagger. While pandas by default represents these as integers, we'll use strings for better swagger handling. An important note however, this only works for a subset of possible Pandas DataFrame orients ('records', 'index', 'columns', 'table'), due to current limitations with conforming to OpenApi version 2. v3 support will come later, at which point this handling should work for all orients.