watch for leading negative on number in mathematica parser

issue identified at http://stackoverflow.com/questions/26249993/tex-commands-as-input-in-sympy/26261553

Before:

```
>>> mathematica('-6x')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sympy\parsing\mathematica.py", line 8, in mathematica
    return sympify(parse(s))
  File "sympy\core\sympify.py", line 319, in sympify
    raise SympifyError('could not parse %r' % a, exc)
sympy.core.sympify.SympifyError: Sympify of expression 'could not parse u'-6x''
failed, because of exception being raised:
SyntaxError: invalid syntax (<string>, line 1)
```

After

```
>>> mathematica('-6x')
-6*x
```