pwiz generates invalid model name for tables starting with numbers
Modified copy-paste of process to produce error:

```
$ python -m pwiz -e mysql XXXXX -H XXXXX -P -u dbadmin -t 1337_table > db_models.py
Password: 
$ python
Python 3.5.1 (v3.5.1:37a07cee5969, Dec  5 2015, 21:12:44) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import db_models
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/some/file/location/db_models.py", line 12
    class 1337Table(BaseModel):
             ^
SyntaxError: invalid syntax
>>> 
```
