fix-chebpoly

These changes implement @trefethen's request in #991 that `chebfun.chebpoly()` continue to output coefficients in the MATLAB style and order.  While we're at it, we beef up the test for `chebfun.chebcoeffs()` (which really wasn't testing much of anything, come to find out) and fix some bugs in the computation of second-kind coefficients.