set up install/delete lists for "push" commands from server

It is now possible for the server operator to specify lists of apps that
must be installed or deleted on the client (aka "push installs).  If
the user has opted in, or the device is already setup to respond to
these requests, then fdroidclient will automatically install/delete
the packageNames listed.  This is protected by the same signing key
as the app index metadata.

It generates single XML elements with the data set in the attributes. This
keeps the XML compact and easily extensible, e.g. for adding versionCode,
signingKey, etc as attributes:

    <install packageName="com.fsck.k9"/>
    <install packageName="at.bitfire.davdroid"/>
    <delete packageName="com.facebook.orca"/>

Copyright: 2016 Blue Jay Wireless
Signed-off-by: Hans-Christoph Steiner <hans@eds.org>
closes #177