Allow subclasses of Symbol

[isuruf]

Fixes #123.

@KristianJensen, you can do the following now

import symengine.sympy_compat as symengine

class Wrapper(symengine.Symbol):
    def __new__(cls, name, extra_attribute):
        return symengine.Symbol.__new__(cls, name)

    def __init__(self, name, extra_attribute):
        super(Wrapper, self).__init__(name)
        self.extra_attribute = extra_attribute

# Instantiate the subclass
x = Wrapper("x", extra_attribute=3)
assert x.extra_attribute == 3
two_x = 2 * x
# Check that after arithmetic, same subclass is returned
assert two_x.args[1] is x

[KristianJensen]

That's fantastic. Thank you!

[certik]

Thanks @isuruf !