Panel.resample should default to axis=1
``` python
In [1]: import pandas as pd

In [2]: panel = pd.util.testing.makePa
pd.util.testing.makePanel    pd.util.testing.makePanel4D

In [2]: panel = pd.util.testing.makePanel()

In [3]: panel.resample("W")
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-b71dd2bc83c6> in <module>()
----> 1 panel.resample("W")

/Users/datacaliber/Dropbox/Projects/trading/python/externals/pandas/pandas/core/generic.py in resample(self, rule, how, axis, fill_method, closed, label, convention, kind, loffset, limit, base)
   2138                               fill_method=fill_method, convention=convention,
   2139                               limit=limit, base=base)
-> 2140         return sampler.resample(self)
   2141
   2142     def first(self, offset):

/Users/datacaliber/Dropbox/Projects/trading/python/externals/pandas/pandas/tseries/resample.pyc in resample(self, obj)
     99             return obj
    100         else:  # pragma: no cover
--> 101             raise TypeError('Only valid with DatetimeIndex or PeriodIndex')
    102
    103         rs_axis = rs._get_axis(self.axis)

TypeError: Only valid with DatetimeIndex or PeriodIndex
```

Would expect this to default to the major axis. 
