Refactoring of *Field classes:

Refactoring of *Field classes:
* Made them all implement one common interface (might add more functions to it later)
* Moved MySQLField to DatabaseMysql.php
* Renamed nullable() to isNullable()
* Removed maxLength() from:
** SQLiteField: makes no sense
** MySQLField: doesn't do what people may think, useless for this class' purpose of assisting querying the DB schema