Join methods for WidePanel

[wesm]

e.g.

def create_panels_join(a, b):
""" return a join of the 2 panels """
d = dict([ (i,(a[i].T.join(v.T)).T) for i, v in b.iteritems() ])
return p.WidePanel.fromDict(d)

this is a 'more' correct version, but have to assure that a[i] is not None

def create_panels_join(a, b):
""" return a join of the 2 panels """
items = set(a.items) + set(b.items)
WidePanel.fromDict(dict([ (i,(a[i].T.join(b[i].T)).T) for i in items ]))

[wesm]

This got done by @jseabold on 9/8. Still need to implement multi-join