Added version checker interface
inserted setUserAgent in each HTTP request
added APIVersion when call NewRegistry
Insert version checkers when call NewRegistry()
added client's kernel version
Removed an unnecessary error check.
Removed an unnecessary nil assignment
format in the user agent header should follow RFC 2616
reduce the number of string copy operations.
documentation.
Rename: VersionChecker->VersionInfo.
versionCheckers()->versionInfos().
Merge pull request #1064 from monnand/156-user-agent-header

Add user agent when calling the registry