Count is the number of non-null elements

Fixes https://github.com/ContinuumIO/blaze/issues/443 
```
In [5]: from blaze import Symbol

In [6]: t = Symbol('t', '{name: string, amount: int}')

In [7]: t.name.count
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-7-56df8cac0c4b> in <module>()
----> 1 t.name.count

/Users/pcloud/Documents/code/py/blaze/blaze/expr/expressions.py in __getattr__(self, key)
     89     def __getattr__(self, key):
     90         try:
---> 91             return object.__getattribute__(self, key)
     92         except AttributeError:
     93             if self.fields and key in self.fields:

AttributeError: 'Field' object has no attribute 'count'
```