Fixed #115. Removed changes in add, updated changes in mul

Restored the print of add, made changes in print of mul.

Fixed the error of -x/x.
Current output:
In [1]: from csympy import *

In [3]: var("x y z")
Out[3]: (x, y, z)

In [4]: x
Out[4]: x

In [5]: -x
Out[5]: -x

In [6]: -1*x
Out[6]: -x

In [7]: -2*x
Out[7]: -2x

In [8]: -x/x
Out[8]: -1