Add example notebook sparse_covariance_est from CVX.

This example was ported from the CVX example "Sparse covariance
estimation for Gaussian variables" by Joelle Skaf, 04/24/2008.

Porting was done by Judson Wilson, 05/22/2014.

Overall, code is similiar, besides the port changes, although several
issues were fixed. The original problem thresholded elements of the result
to be 0 if they were less than 1e-4. The correct way is to threshold
the absolute value of the element. After that, the value of alpha
had to be greatly reduced to cause true sparsity, since all the
thresholded values were large-magnitude negative values.

As an improvement, plots for 3 different values of alpha were added.
Add example notebook clock_mesh from CVX.

This example was ported from the CVX example "Sizing of clock
meshes" by Lieven Vanderberghe and others.

Porting was done by Judson Wilson, 05/26/2014.

Overall, code is very similiar, almost a straight port.
Presently execution is about 100 times slower than on CVX, so
I had to reduce the number of test points, npoints, by a factor
of 2, and it still takes forevor. Also, the solutions are a
little bit different, and the constraints aren't exactly met.

Also made some slight cosmetic tweaks to the text on the graphs.