Document django_authgroupex.auth.AuthGroupeXBackend._update_groups

This function is quite complex to understand so add some comments to
help reading.
Show user fields on the welcome page of the dev site

This helps understanding which fields from the auth-groupe-x request are
mapped to User object.
Improve Makefile

* Remove build directory with "make clean"
* Introduce "make resetdb" to initialize the dev database
Fix grpauth field in fake authentification
Show grpauth field in preset account selection, in fake auth endpoint
Forward "group" GET parameter in fake auth

The "group" GET parameter is the name of a requested "GroupeX".  Forward
it to the fake endpoint to ease debugging.
Use group in dev settings
Add examples in README about permission mapping for groups

While at it, fix a minor RST typo