Fixing integral subs

First attempt at fixing integral subs.

1. If old, new in eq.subs(old, new) are both functions of the integration variables, then substitution takes place irrespective of whether eq is a definite or indefinite integral.

2. If it is a definite integral, then this is followed, https://code.google.com/p/sympy/issues/detail?id=3829

3. If it is indefinite, and there is a linear transformation, then it is xreplaced.