Fix nullable mapping to a degree, other fixes

We needed to invert mysql and sqlite nullable columns to get the correct
value, whoops. Clean up the Config interface by removing unneeded
functions and preparing for the join future. Add some support to mixin
for nullable columns, next commit will finish the support. Alias
map[string]interface{} to Attributes to make UpdateAttributes calls
nicer to read. Add nullable-ity to reflectScanner's and move them and
planner things out to planner.go. Also move retrieval methods out to a
retrieves file.
Finish null checks, add Formula's to scopes

You can now call IsNull on your mixin'ed structs to find out if a column
was null when retrieved, also add Formula constructors for using sql
functions in conditions.