Add back "0 + 1*x" prints as "x" funtionality
change .toPrecision() in if-statements to Number()
add 'x - -2' -> 'x+2' func.
add '2*3^2' -> '2(3)^2' func.
Make prettyprint support options

- prettyprint takes in an 'options' argument
- split node symbol and node operator toString() overrides
- add option to toggle simplification
- add toTex() support
add test! remove old test file
make 'a + b*x' -> '(1.00) + (2.00)*x' the default, split Operator into subfunctions
remove LaTeX from tests, add more test for default behavior
add 1 more test for prettyprint
Merge pull request #2 from plotly/etienne-print

Add back "0 + 1*x" prints as "x" functionality