Merge pull request #159 from maniac103/for-upstream

More fixes
Update README.markdown
Fix potential NPE.
Workaround broken scheme check in Gingerbread's DownloadManager.

It rejected the https scheme even though it can handle it just fine.
Workaround this by copying in the respective internal code and use it
for API < 11.
Don't crash the app when clicking invalid links.
Avoid potential NPE.

Seemingly the gist user field is not filled out in some cases.
Basic support for relative URLs (links and images) in READMEs.
Also support relative URLs in locally rendered markdown.
Fix potential NPE.