symfun: major clean up dropping fragile cludges b/w sym and symfun

Perhaps S('f(x,y)') didn't work in early SymPy?  But its fine in 0.7.5
so we don't need all of this.  Much simpler code now.

syms is still a bit cludgy because it needs to assign x and y back to
the workspace, but this is no worse than other places in syms!

update symfun docs, minor other updates.

Added some new tests for symfun's w.r.t. whitespace and variable ordering.
symfun: display shows the variables of the symfun

Fixes #127.  Adds a "private" method (which isn't really private).
Putting it in private/ doesn't help (display.m only sees @sym's
version).  Someday classdef will deliver us :-)
Merge pull request #128 from cbm755/drop_symfun_hacks

Drop symfun hacks, improve display