Adding custom attribute to use in Airflow's internal DB and a flask session fix

[alexkruc]

Hi,
I have created a pull request that does the following:

• Small cosmetic change to README.rst file - there are code blocks now.
• Made the changes to resolve the double login in the first time - as mentioned in the following case: https://issues.apache.org/jira/browse/AIRFLOW-1021
• Added the ability to specify a custom airflow_user_attr - the user that will be saved inside Airflow's DB.
  In our case, the LDAP server is returning the full user-dn when calling group_member_attr. In order to properly compare the values, we need to save the user-session as the user-dn itself.
  In this PR I added this capability and also updated the documentation for this feature. Note that if this option is not set, the code will behave normally and it's backward compatible.

I would appriciate your review and merge if possible,
Alex.

[alexkruc]

@f-ld I closed the pull request by accident and re-opened it. Did you have a chance to go over it?
I will appreciate your review and merge if possible :)