Fix for series expansions in products of symbols with assumptions

I've been running into problems using series expansion of functions when doing the expansion in terms of a sympy expression that is a product of symbols with assumptions (of positivity). If no assumptions are specified the expansion works as expected, but when assumptions are added the series expansion fails with a traceback error message. This notebook demonstrates the issue:

http://nbviewer.ipython.org/gist/jrjohansson/bf7862198171b16d0ff9

This PR is an attempt to fix this issue by introducing a dummy variable in which the expansion is calculated, and then substitutes back the original expression before returning the result.
I see a different error here in the master:
```
In [4]: s2 = series(exp(x), x, n=8)
...
TypeError: Variables are not symbols, got [a*b]
```