Mrvasympt

MrvAsympt algorithm to find asymptotic expansion.
Are you sure your function actually works as expected here?  I'm not sure:
```
In [3]: log(x+1).aseries(x,logx=y)
Out[3]: 
 1      1      1      1     1        ⎛1        ⎞
──── - ──── + ──── - ──── + ─ - y + O⎜──; x → ∞⎟
   5      4      3      2   x        ⎜ 6       ⎟
5⋅x    4⋅x    3⋅x    2⋅x             ⎝x        ⎠
```

I think, it may be a good idea to add test (less trivial) for aseries with log.