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)

Note that the `CharField` contains the hashed password, and thus the password could theoretically be of any length, unless we limit it separately in the signup/login forms.