Added support for alternative variables (fixes #19)

[jcrist]

• 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.

[jcrist]

Fixed:

• eps added for complex types. Fixes the complex constructor bug in a more elegant way
• Poly.var attribute changed to type string. Allows use of jω or other symbols, as well as multiple character variables

I rolled back the changes to the printing function, as they were outside the scope of this enhancement. I'll open another issue where we can discuss the best way to implement compact printing.