ENH: provide squeeze method for removing 1-len dimensions, close (GH #2544)

```
In [1]: p = Panel(randn(3,4,4),items=['ItemA','ItemB','ItemC'],
   ...:                        major_axis=date_range('20010102',periods=4),
   ...:                        minor_axis=['A','B','C','D'])
   ...:

In [2]: p
Out[2]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 3 (items) x 4 (major_axis) x 4 (minor_axis)
Items axis: ItemA to ItemC
Major_axis axis: 2001-01-02 00:00:00 to 2001-01-05 00:00:00
Minor_axis axis: A to D

In [3]: p.reindex(items=['ItemA']).squeeze()
Out[3]: 
                   A         B         C         D
2001-01-02 -1.182806 -1.835015 -0.008415  1.665945
2001-01-03 -0.347332 -0.257820  0.167772 -1.193518
2001-01-04 -0.502643  0.055733  0.733034 -1.070536
2001-01-05 -1.900588 -1.728706  1.060564 -1.138837

In [4]: p.reindex(items=['ItemA'],minor=['B']).squeeze()
Out[4]: 
2001-01-02   -1.835015
2001-01-03   -0.257820
2001-01-04    0.055733
2001-01-05   -1.728706
Freq: D, Name: B
```