Fix for #45-Check validity of username during installation.

Hi @ruimalheiro 

This fixes #46 - Check validity of username during installation.  

We now validate the following:

* username length max of 75 characters
* password length max of 128 characters
* username allowed characters are "letters, numbers and @/./+/-/_ characters" based on `django.contrib.auth.models.AbstractUser`

See attached screenshot for an example.

/cc @aronasorman 

![ka-lite-windows-installer-2014-10-23a](https://cloud.githubusercontent.com/assets/175580/4751952/8758ffa4-5aa4-11e4-870d-de7c62e84e18.png)

This limit is for the `password` field and is different from the `username` field above.

The password limit of 128 characters I took from [password = models.CharField(_('password'), max_length=128)](https://github.com/learningequality/ka-lite/blob/master/python-packages/django/contrib/auth/models.py#L200).