Refactor Database Declarative Classes

Refactored the SQLAlchemy declarative base classes so they appear in a single module. Changed the MySQL driver. All master_image files successfully loaded form this version.

ephem/build_master_table.py:
	- Refactored the MasterImages class into database_interface.py. Changed the __init__ method into a load() method and updated the code accordingly. Tested on a live DB and works fine.
    - Switched from a mysql connection string (which implied a mysqldb backend) to a pymysql connecion back end.
    - Updated get_fits_file to handle `linear` png files.

ephem/database_interface.py:
    - Switched from a mysql connection string (which implied a mysqldb backend) to a pymysql connecion back end.
    - Added a MasterFinders table.

ephem/run_ephem.py
    - Switched from writing to the Finders table to the master_finders table.