Allows field labels to be set in config

This makes it relatively easy to e.g. localise them, by simply in your app config saying

```python
from flask.ext.babel import lazy_gettext as _

SECURITY_LABEL_EMAIL = _("Email Address")
SECURITY_LABEL_PASSWORD = _("Password")
```

I guess the problem with #155 was that the forms classes were constructed outside an application context. I solved that by creating a proxy object that holds back the actual string realisation until it's needed.

This makes #18 also relatively trivial now.