Incorrect attribute name in CF test check_data_types
Whenever I run the CF checker, I get the following message at the start of the results:

```
The following exceptions occured during the cf checker (possibly indicate compliance checker issues):
cf.check_data_types: NetCDF: Attribute not found
```

Running it in verbose mode also prints a stack trace which points to line 271 of cf.py:

```
    for k, v in ds.dataset.variables.iteritems():
        if v.datatype not in [np.character,
                             ...
```

Assuming v is a NetCDF4.Variable object, it should be `v.dtype` instead of `v.datatype`.

diff --git a/compliance_checker/cf/cf.py b/compliance_checker/cf/cf.py
index a8e805e..8f1a629 100644
--- a/compliance_checker/cf/cf.py
+++ b/compliance_checker/cf/cf.py
@@ -268,7 +268,7 @@ class CFBaseCheck(BaseCheck):
         total = len(ds.dataset.variables)
 
         for k, v in ds.dataset.variables.iteritems():
-            if v.datatype not in [np.character,
+            if v.dtype not in [np.character,
                                   np.dtype('c'),
                                   np.dtype('b'),
                                   np.dtype('i4'),
['compliance_checker/cf/cf.py']