[IMP] fields: optimize low-level reading of fields (in columns)

Avoid post-processing like name_get() on many2one fields, since the result is
simply put in cache. The method read() will retrieve values from the cache,
which will post-process them again!
Merge pull request #16 from odoo-dev/master-apiculture-no_name_get-rco

[IMP] fields: optimize low-level reading of fields (in columns)