Merge pull request #32 from sschuberth/develop

DownloadFileTask: Support redirection across schemes (e.g. HTTPS to HTTP)
Update README.md
DownloadFileTask: Recursive resolving of redirects across schemes
AsyncTaskUtils for switching between execute and executeOnExecutor
Merge pull request #34 from bitstadium/feature/recursiveredirect

DownloadFileTask: Recursive resolving of redirects across schemes
Merge branch 'feature/taskexecute' into develop

Conflicts:
	src/main/java/net/hockeyapp/android/tasks/DownloadFileTask.java
If the file size is unknown, says so instead of showing it as "0.0 MB"
Generalize DownloadFileTask a bit so it can be more easily sub-classed
Add a GetFileSizeTask to determine the file size of externally hosted APKs