If not exist insert

PR for if_not_exists against master
It's a little weird that this doesn't throw an exception... there's no indicator that the LWT failed.  I'd expect something like:

```
try:
    TestIfNotExistsModel.if_not_exists().create(id=id, count=9, text='111111111111')
except LWTException as e:
    # handle failure case
    print e.existing # existing object
```

Here's the results from a simple test.  

![image](https://cloud.githubusercontent.com/assets/337872/4123268/78db5166-32cc-11e4-8c1d-b0e1d9b69ee8.png)

This seems like it should fail in the connection.execute.  Thoughts?