Huge performance gain in Array Controller

Performance gain is done by not calculating max(array_keys()) or
whatever to get next sequence id. Now we simply store it in max_id
attribute in controller class and increment on save.
For me (inserting ~6000 records in session based model+controller) it
gave performance gain of ~300-400% !! :)

Removed some unused variables and methods from controllers. Please check
if I'm not mistaken, but these looks unused or deprecated.

Also fixed some typos in readme.md.
Should initially calculate max_id in special case
Merge pull request #317 from DarkSide666/master

Huge performance gain in array controller