[MRG] epic bin merging algorithm

Merging specific bins along any axis has never been this easy!

```python
    def merge_bins(self, bin_ranges, axis=0):
        """
        Merge bins in bin ranges

        Parameters
        ----------

        bin_ranges : list of tuples
            A list of tuples of bin indices for each bin range to be merged
            into one bin.

        axis : int (default=1)
            The integer identifying the axis to merge bins along.

        Returns
        -------

        hist : TH1
            The rebinned histogram.

        Examples
        --------

        Merge the overflow bins into the first and last real bins::

            newhist = hist.merge_bins([(0, 1), (-2, -1)])

        """
```

![binmerge1d](https://f.cloud.github.com/assets/202816/1061954/413db1b8-1216-11e3-9828-2f6845fb4b05.gif)

![binmerge2d](https://f.cloud.github.com/assets/202816/1061955/449a6e00-1216-11e3-9f6e-bf41ecaa71a9.gif)