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% !! Time drop down from ~55 seconds to 16 seconds.
Calculation of max_id using max(array_keys()) can happen, but only once (initially) in case we somehow set data in model / controller without calling save() method.

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.