Added support for alternative variables (fixes #19)

- Added support for optional variable parameter in constructor. 
- Added string function to get string representation without 'Poly(...)' appended. 
- Improved printing function, now it: 
    - formats coefficients up to 4 decimal places (i.e. 1.123456 -> 1.1234, and 1.00005 -> 1)
    - removes unnecessary powers (i.e. x^1 is just x)
    - removes unnecessary coefficients (i.e. 1x is just x)
- All operations involving two polynomials now must have the same 'var' attribute (error is thrown otherwise)
- Added tests for new functionality, and updated the README.

I also fixed a small bug preventing complex polynomials from being created. There's no default comparison between a complex and a real (complex < real will error). Changed line 22 to fix that. Poly([1 + im, 1 - im]) now is valid.

Let me know what you think.