Use "(No column name)" as column name when as_dict and no column name

I propose that we modify pymssql to use a column name of `"(No column name)"` when the user uses `as_dict=True` and then does a query which returns output columns that are not named -- e.g.:

```sql
SELECT MAX(x) FROM (VALUES (1), (2), (3)) AS foo(x)
```

```python
cursor = conn.cursor(as_dict=True)
cursor.execute("SELECT MAX(x) FROM (VALUES (1), (2), (3)) AS foo(x)")
print(cursor.fetchall())
# Outputs:
# [{}]
# Whoa, what happened to MAX(x)?!?!
```

Currently, we don't add the column to the result row if it has no name. This confuses people. See [this mailing list thread](https://groups.google.com/forum/?fromgroups#!topic/pymssql/JoZpmNZFtxM) for an example.

I have a more detailed example at: https://gist.github.com/msabramo/8319097

I chose `"(No column name)"` because this is what SQL Server Management Studio uses.

So the output of the above Python snippet with this change is:

```
[{'(No column name)': 3}]
```