* Set the correct client signature algorithm

Before, the localtime field was set to a wrong datatype (DateTime
instead of TimeZoneDataType), and didn’t contain the local timezone
offset, but the current localtime.

Previously this was hardcoded to sha1. Fix this by introducing a
new URI field in the SecurityPolicy classes carrying the correct
URIs.

This fixes complaints by some UA clients that match requested
security profiles against the server response and warn the user.

The method 'to_string' was changed and 'from_string' was created. The function 'string_to_val' was changed to use 'from_string' for LocalizedText.

The Locale property was changed in the LocalizedText class. It is now implemented in the same way as the Text property with a corresponding setter.

Fix for issue #18