Pretty Printing of -1x: Fixes #115

Fixed the printing of -1x

Before:
-------
In [1]: from csympy import *

In [2]: var("x y z")
Out[2]: (x, y, z)

In [3]: -x
Out[3]: -1x

In [4]: -2*x
Out[4]: -2x

-----------------------------

After:
------
In [1]: from csympy import *

In [2]: var("x y z")
Out[2]: (x, y, z)

In [3]: -x
Out[3]: -x

In [4]: -2*x
Out[4]: -2x