Remove explicit table and column in Django model fields

[yujiyokoo]

This removes the need to be explicit about table and column in Django models. Instead of:

class User(models.Model)
    name = EncrypytedText(eql_table="users", eql_column="name)

the field can be simplified to:

class User(models.Model)
    name = EncryptedText()

It is still possible to provide the table and column names in case you have different names, but they are now prefixed with eql_.