Changes from update_attributes to assignment

[vesan]

Marks position column as not attr_accessible (like it is in many apps)
and sets the column by directly setting the attribute.

Fixes issue #50

I don't know if there was a reason for using update_attributes!, I couldn't
find any. This patch adds set_list_position public method. It could also
be private but there are few tests that used update_attributes! which I
converted to use the new set_list_position method.

This way of fixing the problem doesn't make acts_as_list dependent of
attr_accessible like #51
which is nice considering strong_parameters seems to be the future.

[itsNikolay]

Good pull request!
Voting for merge it.
Without the request, I getting brainblowing issue when I use:

attr_accessible :position, as: :admin

[swanandp]

This is good, thanks!

[swanandp]

This got merged with #60.

[vesan]

Thanks!