Updated pow_expand to expand integral powers of complex nos

* Updated print for `add` in case of Complex numbers
* Added `pow_number` function
* Added a test for checking running time
Called pow_number in pow. Updated to handle negative power cases.
Removed redundant code blocks and replaced with a function

Added a new function, pow_complex which contains the code
corresponding to complex^integer
Updated function definition for `pow_complex`

Now `pow_complex` takes `exp_` as `const Integer` as opposed to
`RCP<const Integer>` in the former version
Merge pull request #264 from sushant-hiray/complex-pow

Expand integral powers of complex nos