Add support for "IS"/"IS NOT"

Support use of "IS" and "IS NOT" similarly to "IS NULL" etc. Currently, limited to using "=" to check for equality, which generally has different behaviour when comparing null values.

For example, 

```
c1 = Field("foo").is(True)
c2 = Field("foo").isnot(True)

self.assertEqual('"foo" IS TRUE', str(c1))
self.assertEqual('"foo" IS NOT TRUE', str(c1))
```


I'm happy to pick this up.