Visiting an NDArray shouldn't create a new expression id.
Prettier string representations for most nodes.

The default debug string is useful for... debugging.

It's a little too much information for most purposes.  Change
the default representation to be a bit prettier.
Add missing method.
Don't show local input keys in pretty strings.
Merge pull request #128 from spartan-array/pretty-print

Prettier string representations for most nodes.
Merge branch 'master' into fix-ndarray

Conflicts:
	spartan/expr/ndarray.py
Merge branch 'master' into optimization

Conflicts:
	spartan/expr/base.py