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.
remove practice of evaluating factorial as 0 for negative numbers
it is infinity for negative integers and remains unevaluated for non-integers. fix gosper test, which had a missing term.
fix string representations
doctest fix
use complex infinity for factorial of negative integers
note that the factorial change fixes #2703 on github
Added test that checks for LaTeX representations.
Fixed tests for Python 3 unicode.
Fix github's issue #2725
Add mpmath's translation for Abs
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.
Added Vectors and Dyadics to IPython printing.

The Vector and Dyadic objects in the mechanics module have the capability to
print as LaTeX. I've added these to the init_printing `_can_print_latex` check
and now they render in the notebook properly. This fixes issue #2584.
Changed to \cdot for separating unit vectors and scalars.
Removed \cdot and went with no space.
Merge pull request #2687 from moorepants/mechanics-ipython

Mechanics Vectors and Dyadics print in LaTeX with init_printing()
Fixes Vector latex printing in issue #2727.
Merge pull request #2728 from moorepants/vector-printing

Fixes Vector latex printing in issue #2727.
Merge pull request #2726 from skirpichev/gh2725

Fix github's issue #2725
Merge pull request #2707 from hacman/factorial

remove practice of evaluating factorial as 0 for negative numbers