Add Pandas, Persist Data, Create Observation model

New
———
- Added pandas library for future time series data analysis
- Created ‘Observation’ model that is persisted in a MySQL database
- Persist data: if the data exists in our local database, we don’t have
to call Yahoo and Quandl for it. If not, download the data and store it
for future use.

TODO
——
- Keep track of the number of times stock data is accessed
Add PIP requirements