Unit unpickling fails for UnrecognizedUnit

As originally noted in the post-merge discussion of #792:
```
In [56]: t = Table([[1]])
In [57]: t['col0'].unit = 'blah'  # would succeed for unit = 'm'
In [58]: tpp = pickle.loads(pickle.dumps(t))
ERROR: AttributeError: 'NoneType' object has no attribute '__dict__' [pickle]
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-58-1f551d7490ab> in <module>()
----> 1 tpp = pickle.loads(pickle.dumps(t))

/Users/aldcroft/anaconda/envs/astroquery/python.app/Contents/lib/python2.7/pickle.pyc in loads(str)
   1380 def loads(str):
   1381     file = StringIO(str)
-> 1382     return Unpickler(file).load()
   1383 
   1384 # Doctest

/Users/aldcroft/anaconda/envs/astroquery/python.app/Contents/lib/python2.7/pickle.pyc in load(self)
    856             while 1:
    857                 key = read(1)
--> 858                 dispatch[key](self)
    859         except _Stop, stopinst:
    860             return stopinst.value

/Users/aldcroft/anaconda/envs/astroquery/python.app/Contents/lib/python2.7/pickle.pyc in load_build(self)
   1222         if state:
   1223             try:
-> 1224                 d = inst.__dict__
   1225                 try:
   1226                     for k, v in state.iteritems():

AttributeError: 'NoneType' object has no attribute '__dict__'
```