Major API change for TnyMimePartSaveStrategy & TnyMimePartSaver

[Philip Van Hoof <spam pvanhoof be>]

I switched the _save and _perform_save of the TnyMimePartSaveStrategy
because in design pattern documentation and samples it's like how it's
now.

By that I mean that the strategy has the "perform_it" method, and the
subject as the "it" method.

I know it's caring about the little things, but it's also better to get
it right now, than to change it post 1.0 API.

-- 
Philip Van Hoof, software developer at x-tend 
home: me at pvanhoof dot be 
gnome: pvanhoof at gnome dot org 
work: vanhoof at x-tend dot be 
http://www.pvanhoof.be - http://www.x-tend.be