Support authentication data in uReport

We need this for 'machine id' element which should be present only if
users wishes to it.

The authentication entries are string based, key value pairs (JSON
string elements).

The entries are enclosed in an envelope object called 'auth'.

Example:

"auth" : {   "machineid" : "deadbeaf"
         }

Fixes #171

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Martin Milata <mmilata@redhat.com>

mmilata: s/custom/auth/g, python bindings
tests: add a test for uReport auth data

Related to #171

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Martin Milata <mmilata@redhat.com>

mmilata: python test
Merge pull request #183 from abrt/gh171_ureport_auth

uReport authentication data