metadataCurrent() to check if need_download_mdFile

Using the yumrepo metadataCurrent() to check if needs to download mdFile
It must not check withinCacheAge with the localMDFile.
Merge pull request #1 from ronaldosaheki/master

Use metadataCurrent() to check if need_download_mdFile
remove unused 'maxhostfileage' parameter
update README for github migration

Let's use git-tag to release the software, then we can get
zip/tar.gz files from https://github.com/crook/yum-axelget/releases

Github issue page is also a good channel for bug report/discussion.
https://github.com/crook/yum-axelget/issues

This patch also added "Contribution" in README.md

Signed-off-by: Amos Kong <kongjianjun@gmail.com>