Add configuration check for sqlite
Thanks, good idea!

Some operating systems don't have sqlite3 module within python3.
without this commit, they will get unclear error message like this:

```
File "/usr/local/lib/python3.4/site-packages/peewee.py", line 3371, in
connect
    **self.connect_kwargs)
  File "/usr/local/lib/python3.4/site-packages/peewee.py", line 3609, in
_connect
    conn = sqlite3.connect(database, **kwargs)
AttributeError: 'NoneType' object has no attribute 'connect'
```

Those users who encounter this ImproperlyConfiguredError
may have to install sqlite dependencies and recompile their python3
