Fix 2 issues with new-line. (issue  #1018)

— Apply patch from @glazedkoala. Fix the issue where Label is reused
with new-string being smaller than previous one. With fix character is
not displaced any more.

— _width logic was broken for long time. If CCLabelBMFont is created
with some width say N, then expectation is to see new-line once string
is out-of bound.
     This effect is more visible where user is updating label
dynamically (like appending single character)