toString() it's not working with negative numbers

[magdy-zon]

Hi my friend!

I was working perfectly with your library until I had a trouble with a simple example:
When I was trying to parse '-1' I do:

>>> parser.parse('-1').toString()

Shows an error:
Traceback (most recent call last):
File "", line 1, in
File "/Library/Python/2.7/site-packages/py_expression_eval/init.py", line 185, in toString
nstack.append('(' + f + n1 + ')')
TypeError: cannot concatenate 'str' and 'float' objects

Thanks for read my issue and have good code lines.

[flotwig]

A similar issue happens when you try to use toString() on an expression that uses functions, it says that TypeError: must be str, not float on line 187.