snf_django: Fix escaping of control characters

Fix escaping of control characters from string and unicode objects. The
function 'safe_string' has been renamed to 'escape_ctrl_chars'.
Also, this function has been applied to the output of 'pprint_table'
utility function and has been added as a method of the 'SynnefoCommand'
class.
astakosclient: Use 'repr' when logging

Use 'repr' instead of 'str' when logging the response body.
astakosclient: Human readable details message

In case of QuotaLimit exception, fill 'details' with a human readable
message and put the actual JSON response in the 'response' variable.
Synnefo UI shows the 'details' field of an exception to the user so it
has to be human readable.
astakosclient: Fallback to json module

If 'simplejson' module is not available fallback to 'json'.