Create, or get if exists, a Symbol.
        
        Parameters
        ----------
        name : str
            A symbol's name is a primary key, used across
            the Trump ORM.
        description : str, optional
            An arbitrary string, used to store user information
            related to the symbol.
        units : str, optional
            This is a string used to denote the units of the final
            data Series.
        agg_method : str, optional
            The aggregation method, used to calculate
            the final feed.  Defaults to priority_fill.
        overwrite : bool, optional
            Set to True, to force deletion an existing symbol.
            defaults to False.
            
        Returns
        -------
        Symbol

def create(self, name, description=None, units=None,
               agg_method="priority_fill", overwrite=False):
        """ Create, or get if exists, a Symbol.
        
        Parameters
        ----------
        name : str
            A symbol's name is a primary key, used across
            the Trump ORM.
        description : str, optional
            An arbitrary string, used to store user information
            related to the symbol.
        units : str, optional
            This is a string used to denote the units of the final
            data Series.
        agg_method : str, optional
            The aggregation method, used to calculate
            the final feed.  Defaults to priority_fill.
        overwrite : bool, optional
            Set to True, to force deletion an existing symbol.
            defaults to False.
            
        Returns
        -------
        Symbol
        """
        sym = self.try_to_get(name)

        if sym is not None:
            if overwrite:
                print "Deleting {}".format(sym.name)
                self.ses.delete(sym)
                self.ses.commit()
            else:
                msg = 'Symbol {} already exists.\n' + \
                      'Consider setting overwrite to True.'
                msg = msg.format(name)
                raise Exception(msg)

        sym = Symbol(name, description, units, agg_method)
        
        self.ses.add(sym)

        print "Creating {}".format(sym.name)
        sym.add_alias(name)

        sym.handle = SymbolHandle(sym=sym)
        self.ses.commit()

        return sym