Continuity of Integrated Piecewise Function. Issue 3732.

The added code is currently in comments. It works for indefinite integration and cases of definite integration where `offset` does not reach `oo`.

https://code.google.com/p/sympy/issues/detail?id=3732

Currently,

>>> from sympy import *
>>> from sympy.abc import x
>>> q = Piecewise((0, x<-1), (1, x<0.5), (0, True))
>>> integrate(q, x)
Piecewise((0, x < -1), (x, x < 0.5), (0, True))


Desired,

>>> from sympy import *
>>> from sympy.abc import x
>>> q = Piecewise((0, x<-1), (1, x<0.5), (0, True))
>>> integrate(q, x)
Piecewise((0, x < -1), (x, x < 0.5), (1.5, True))