Experimental support for MatPow symbolic matrix powers

This is supposed to allow A^n where n is symbol.
matpow: replace test
python_header: try to write MatrixExpr as Matrix on return

This should allow "subs" to work with "A^n".
Merge remote-tracking branch 'origin/master' into matpow
mpower: note upstream sympy fix
MatrixExpr hack: make 0.7.5 specific
Merge remote-tracking branch 'origin/master' into matpow
add entry to NEWS, document 0.7.5 special case
mpower: clarify a comment

this is not the right place for this, but its got a FIXME attached
so good enough ;-)
prelim vpa support
minor cleanup
backslash with vpa raises warning

b/c its not consistent with double for over determined systems
Merge branch 'backslash' into vpa
fix typo
use sympy's "N" to make Floats

allows `vpa('pi')` to work.
vpa quieter tests
Support symbolic matrix creation: sym('a', [2 3])

still work-in-progress: lots of things don't work and I'd like to have
it closer to SymPy's MatrixSymbol in the [2, 3] case (already using that
in the [n, m] case with symbolic size).

Probably needs better MatrixExpr doit() support upstream, because of
the "as_explicit()" workaround I'm currently using in python_header.  That
does something horrid to MatrixSymbols with [2 3] size.
Merge remote-tracking branch 'origin/master' into matpow
reformat NEWS file for next release
Merge remote-tracking branch 'origin/master' into matpow

Conflicts:
	NEWS