BUG: Make dict iterators real iterators, provide "next()" in Python 2
Currently, `compat.itervalues` and friends are not real iterators (under Python 3): compare

```
In [3]: next(six.itervalues({1:2}))
Out[3]: 2
```

with 

```
In [4]: next(pd.compat.itervalues({1:2}))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-177f5d3e9571> in <module>()
----> 1 next(pd.compat.itervalues({1:2}))

TypeError: 'dict_values' object is not an iterator
```

This PR fixes this (it drops support for `**kwargs`: although it is supported by `six`, I fail to see what's the utility, and it's anyway apparently not used in the `pandas` codebase - but if I'm missing something, it is trivial to reintroduce it) and provides `next()` in Python 2.
