Use IpAddr datatype consistently for IP addresses

[tpikonen]

Fixes always failing comparisons of cached IPs to IPs from address plugins. The cache was returning an address string, plugins IpAddr objects.

• Serialize IpAddr objects (to cache) with IpAddr.__str__(), using repr() of list [self.v4, self.v6]
• Only serialize IpAddr objects (not strings) to cache
• Read IpAddr objects from cache with ast.literal_eval()
• Delete cache file when it cannot be read without errors. This applies to cache files written by previous versions of ddupdate.

[leamas]

hm... that's quite a bug found and fixed. Thanks. Filing issue #19 as reference