Add pk coercing feature to QuerySelectField

So I'm running into a form field validation problem when the formdata provides an int value for my pk (angularJS does that when posting object). Adding coercing (defaulted to text_type) would allow integer pk and would prevent https://github.com/wtforms/wtforms/blob/master/wtforms/ext/sqlalchemy/fields.py#L85 from failing when the submitted formdata is an int.

Usage:
```python
from wtforms.form import Form
from wtforms.ext.sqlalchemy.fields import QuerySelectField

class OptionForm(Form):
  options = QuerySelectField("Options", query_factory=option_q_factory, coerce=int)  

```