Upgrade to Sympy 0.7.3

incompatability changes:
    sympy.Ylm -> sympy.Ynm
    sympy.Expr.as_coeff_terms -> sympy.Expr.as_coeff_mul
    sympy.Symbol('x', dummy=True) -> sympy.Dummy('x')

improvements:
    sympy.solve
Exp should be a _MPMathFunction
install sympy from git in travis-ci tests
fix bug in Real.__cmp__
remove Integer.get_sort_key hack
refactor Log to give more accurate results
add test for large Integer comparison bug
add SympyFunction.prepare_mathics to fix SphericalHarmonicY
monkeypatch sympy.Expr._expand_hint bug
add basic Piecewise and fix piecewise Sum bug
Merge pull request #157 from sn6uv/sympy073

Sympy 0.7.3