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.
Fix notebook cvx_examples/circuit_design/clock_mesh

Steven Diamond pointed out a bad double inequality constraint
of the form:

exp <= exp <= exp

which CVXPY will not fail on, but does nothing, so I fixed
this, and upped the number of optimization points to 50
to get results that match the CVX example within a high
degree of precision.
Add example notebook fir_chebychev_design from CVX.

This example was ported from the CVX example "Chebychev design
of an FIR filter given a desired H(w)" by Almir Mutapcic, 02/02/2006.

Porting was done by Judson Wilson, 05/28/2014.

The largest changes were with basic formatting and language conversion,
and necessary changes to convert complex-valued variables into an
equivelent real-valued representation for CVXPY.
Add example notebook l1_trend_filter_snp500 from CVX.

This example was ported from the CVX example "l1 trend filtering"
by Kwangmoo Koh, 12/10/2007.

Porting was done by Judson Wilson, 05/28/2014.

Very little beyond cosmetic changes and porting. Note that ECOS and
SCS return inaccurate results. CVXOPT returns a good result, but
takes a long time, likely due to a bug Steven Diamond mentioned
where CVXOPT treats sparse matrices as full, at the moment.
Merge remote-tracking branch 'upstream/master' into judsons_examples
Example clock_mesh: Convert to use reshape atom.

The reshape atom was recently added, and makes the constraint
formation in this problem much cleaner. Result is the
same, except for negligible differences in the solution. The
result plots are identical.
Example fir_chebychev_design: LaTeX the title

The H(w) in the title was converted to a LaTeX math expression,
replacing "w" with "\omega".
Example fir_chebychev_design: comment cleanup

Align a comment.
Merge pull request #98 from JudsonWilson/judsons_examples

Judsons examples