IMP: new attritb for save fields readonly explicity

This commit is to allow store fields readonly optionally adding a new keyword(save_readonly). 
According to several comments on the bug https://bugs.launchpad.net/openobject-client/+bug/378824, OpenERP S.A. not fixed ​​this change by a design decision as Oliver says, but I think that is not a good design practice to have hidden fields to obtain the expected behavior.

Note: sorry for bad writing