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)

Good catch @aronasorman!  

This is a technical limitation from the [`AbstractUser`](https://github.com/learningequality/ka-lite/blob/master/python-packages/django/contrib/auth/models.py#L377) class.

I took the 75 characters limit from [here](https://github.com/learningequality/ka-lite/blob/develop/python-packages/django/contrib/auth/models.py#L378) coz I had the `develop` branch active at that time - my bad.  But I should have used the `master` branch where the username limit is 30 characters is used at [username = models.CharField(_('username'), max_length=30...](https://github.com/learningequality/ka-lite/blob/master/python-packages/django/contrib/auth/models.py#L377).

Just had a thought that 30 characters is small if we allow users to use an email address as username, but I will make the change now until the `develop` branch is merged to `master`.