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
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
Added test to assert pretty print of -1x to -x

References pull #119 and original issue #115
Added tests for the following:

-1*x should print as -x
-1*x*y should print as -x*y

Added 4 simple cases to test expected and unexpected behavior.
Updated string compare to == operator

Referring to @certik notes on previous commit.
Updated string compare to == or != appropriately to simplify the
code. (Both run the same underlying code, so simplicity is better
bargain)
Changed rcp(new Integer(-1)) to integer(-1)
Merge pull request #119 from sushant-hiray/pretty_print

Pretty Printing of -1x: Fixes #115