Add -q option (suppress print upon creation) to %macro

Macros are very, very useful and "Matlab" like (as well as other similar
math computing environs). Often I (or my students) use a macro to load
long complex code from a url -- e.g., large data sets, simulated data,
preprocessing of data, special plotting commands, grading routines...
Currently, this requires defining the macro at the end of the notebook
so when the "print upon creation" occurs it doesn't overwhelm the
notebook (except at the end).  The -q option suppresses the print
contents upon creation.

Example with a Matplotlib example:

In[1]:  %macro  tmp http://matplotlib.org/mpl_examples/api/date_demo.py
Macro `tmp` created. To execute, type its name (without quotes).
=== Macro contents: ===
"""
Show how to make date plots in matplotlib using date tick locators and
formatters.  See major_minor_demo1.py for more information on
controlling major and minor ticks
...

In[2]: %macro -q tmp2
http://matplotlib.org/mpl_examples/api/date_demo.py

(nothing)

Perhaps, though, the first line should print -- e.g.,

Macro `tmp` created. To execute, type its name (without quotes).

In the docstraing, I also fixed  a typo (an "as" that should be an "at")
and clarified how to produce an example output.