Constant behaves differently to Dolfin constant

*NOTE*: This was previously tracked under PyOP2, so I am just moving it here.

In Dolfin I can do:

```
Constant( (1.0, 0.0) )
```

which causes the following error when run in Firedrake:

```
TypeError                                 Traceback (most recent call last)
<ipython-input-9-5fa74b24c306> in <module>()
----> 1 velocity = Constant( (1.0, 0.0) )

/usr/local/fenics/ufl/master/lib/python2.7/site-packages/ufl/coefficient.pyc in __init__(self, domain, count)
    114 
    115     def __init__(self, domain, count=None):
--> 116         e = FiniteElement("Real", domain, 0)
    117         ConstantBase.__init__(self, e, count)
    118         self._repr = "Constant(%r, %r)" % (e.domain(), self._count)

/usr/local/fenics/ufl/master/lib/python2.7/site-packages/ufl/finiteelement/finiteelement.pyc in __init__(self, family, domain, degree, quad_scheme, form_degree)
     57             cell = None
     58         else:
---> 59             domain = as_domain(domain)
     60             cell = domain.cell()
     61             ufl_assert(cell is not None, "Missing cell in given domain.")

/usr/local/fenics/ufl/master/lib/python2.7/site-packages/ufl/domains.pyc in as_domain(domain)
    129         return domain
    130     else:
--> 131         cell = as_cell(domain)
    132         if isinstance(cell, Cell):
    133             if cell not in _default_domains:

/usr/local/fenics/ufl/master/lib/python2.7/site-packages/ufl/geometry.pyc in as_cell(cell)
    588         return Cell(cell)
    589     else:
--> 590         error("Invalid cell %s." % cell)

TypeError: not all arguments converted during string formatting
```

(I think this is because `Constant` is not yet implemented in Firedrake and it is trying to give me a UFL `Constant`).