Fix 'PHP Notice:  unserialize(): Error at offset ...' errors

These kind of errors are caused most of the time by:
- unescaped quotes/slashes etc
- miscounted unicode characters

The change in this commit fixes this, though it does mean the cache directory has to be cleared completely (once - on upgrade to the version which includes this fix) for the fix to take effect.
Make cache files identifiable again as with the base64 encoding, there was no way to determine which package the cache referred to which makes selective (manual) deletion difficult.
Added documentation about the use of base64 encoding
Added 'b64' indicator to cache file name
Merge branch 'master' of github.com:YahnisElsts/wp-update-server into Fix-serialization-issues
Merge pull request #11 from jrfnl/Fix-serialization-issues

Base64-encode cached data to fix serialization issues