pithos: Restore XML input in uploads using hashmaps

In version 0.15 the option to create an object by providing the object's
hashmap in XML format, instead of the actual data, was disabled. Restore
this operation by substituting the 'minidom' module (vulnerable to some
security implications) with the 'lxml' module for performing the XML
parsing. Also fix the respective test case.

Close grnet/synnefo#155
Revert "pithos: Restore XML input in uploads using hashmaps"

This reverts commit 11b5b8503f7d85d6abffdfbb5111e21142b45ccb.
ci: Name directory when git-cloning

Explicitly name the directory where we clone a repo; don't rely on the
repo's name.
astakos: Add option to customize sender of emails

Allow changing the sender of an email in the `send_plain` function of
astakos.
astakos: Add test suite for user utilities

Use this test suite to check if `send_plain` function works as intended.
pithos: Refactor statistics subquery

Use CTE to get current population.

Add partial index to versions table.