split off CBase58Data from CBitcoinAddress

Split off features unrelated to addresses from CBitcoinAddress to
CBase58Data, so they can be reused.
Fix bug with accessing vchData[0] when vchData is empty.
Fix typo in CBase58Data::CompareTo
Merge pull request #1 from AbrahamJewowich/cbitcoinaddress

Cbitcoinaddress
Merge pull request #403 from sipa/cbitcoinaddress

keys indexed by address + introduced CBitcoinaddress
Namecoin: fix conflict
Merge commit '03f8b545655bd1a6ee6db7e2e6d14586900cacb1' into namecoin_merge_v0.8
Merge commit 'ca40e581ebcdc85dba15c1e873f5e5aedda45b77' into namecoin_merge_v0.8
Merge commit 'a9ba47101a46533ff0418d6868ebc9bf5c889818' into namecoin_merge_v0.8