Improvements to IPython printing: strings, floats, and ints.

Removed strings from LaTeX printing in IPython as this was causing dictionaries
that contained strings and other valid LaTeX printable types to render. This is
typically not desirable especially if you have a dictionary with only strings,
floats, and integers.

I also added an option `print_builtin` to `init_printing` that allows you to
disable the float and integer LaTeX printing. For example, if you don't want a
dictionary of integer keys and float values to render in LaTeX.

Fixes issue #2672.
Additional notes in doc string about line wrapping.
Added start of tests for print_builtin option.
Added test that checks for LaTeX representations.
Fixed tests for Python 3 unicode.
Tests pass on Python 2/3, IPython 0.13.2/0.1.1.
Merge pull request #2683 from moorepants/ipython-printing

Improvements to IPython printing: strings, floats, and ints.