Theano Piecewise

Add a translation from SymPy.Piecewise to Theano.switch.  This required correcting the translations for inequalities.

```Python
In [1]: expr = Piecewise((0, x<0), (x, x<2), (1, True))
In [2]: from sympy.printing.theanocode import *
In [3]: f = theano_function([x], [expr], dtypes={x: 'float64'}, dims={x: 1})
In [4]: import numpy as np
In [5]: data = np.linspace(-2, 4, 15)

In [6]: f(data)
Out[6]: 
array([ 0.        ,  0.        ,  0.        ,  0.        ,  0.        ,
        0.14285714,  0.57142857,  1.        ,  1.42857143,  1.85714286,
        1.        ,  1.        ,  1.        ,  1.        ,  1.        ])
```

CC @nouiz