Fixing LNT's bugs in chebpolyplot.

Fixes LNT's bugs: 

1. CHEBFUN2.CHEBPOLYPLOT() needs a deprecated message (like Chebfun)
2. Contrary to documentation, it doesn't plot just six curves by default.
3. Title of plots shoudl not contain "Coeffs of". 

One remain (more major) bug: 
4. Chebfun2 does always simplify correctly in the constructor.