add document describing mysql .frm format
implement var_string type format

- used for old varchar columns from mysql < 5.0
cleanup frm parsing api

- rename mysqlfrm/parser.py -> mysqlfrm/binaryfrm.py
- add 'type' property to MySQLView and Table instances
  returned by mysqlview.parse and binaryfrm.parse respectively
- add mysqlfrm.parse method to __init__.py that dispatch to
  either the view or binary frm parsing code
- move the formatting logic to generate a DDL string to the
  format method of each returned object
cleanup .frm format documentation
Merge branch 'feature/unified_frm_api' into develop