Next iteration

[devkral]

Changes:

• split pknames and pkcolumns
• keep mapping between fields and columns, it is lazy
• add related_fields, to select fields for ForeignKeys
• add PKField, a proper, lazy CompositeField for the pk field
• add inherit flag for fields to control inheritance
• lift restriction on no multiple managers in abstract classes
• move meta manager to related model attribute
• remove unused meta fields and use properties for rendundant fields
• fix exclude field
• huge speed improvements
• more speed improvements by making MetaInfo lazy

Many changes for the future. I am so free to add a develop branch