pytave: adding exec and eval commands

[Colin Macdonald]

Hi Mike Miller (and others interested in Pytave),

I played around a bit with Pytave and Symbolic:

1.  I added `pyexec` and `pyeval` commands which mirror the
`exec`/`eval` commands in Python.  See the "execeval" branch here:

https://bitbucket.org/macdonald/pytave/branch/execeval


2.  It was then straightforward to get Symbolic to use these instead of
popen2:

https://github.com/cbm755/octsympy/pull/374

(preliminary!)


3.  Mike: here's a small commit adds "help py":

https://bitbucket.org/macdonald/pytave/commits/2d9db88da67a92401133fec302f9bda3f7aa587e

One could probably do better examples: its just some of the test cases I
used when I was debugging/learning how the `py` command was supposed to
work.  Later, we'll need a way for those commands using `__builtin__.`
to work without that!

best,
Colin