Add an implementation for HttpConnection using Apache HttpClient

Add an implementation for HttpConnection using Apache HttpClient

This change implements the http connection abstraction with the help of
org.apache.http.client.HttpClient. The default implementation used by
JGit is still the JDK HttpURLConnection. But now JGit users have the
possibility to switch completely to org.apache.httpclient. The reason
for this is that in certain (e.g. cloud) environments you are forced to
use the org.apache classes.

Change-Id: I0b357f23243ed13a014c79ba179fa327dfe318b2
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>