Use source version of apache-httpclient

We can't use a repository version because there's no release yet which
contains rev. 1652769. However, this revision is necessary to get SNI with Android <4.2, too.
To avoid packaging a pre-compiled jar, this source lib has been added as a subproject:

apache-httpclient, branch 4.3.5-android, revision 1652769

Fixes #491.