Make ant build the first-time smoother - Fix #365

[goldyliang]

Some minor changes to enable the ant build smoother when building for the first time. If you have other ideas, I would adapt to your view.

Some more detail behind my change can be found at: http://www.goldyliang.net/blog/how-to-build-fbreaderj-android/

Summary:

• Move class NaturalOrderComparator from package org.fbreader.util, to package group.pals.android.lib.ui.filechooser.utils
• Change project.properties files to use same SDK versions. (min/max/target API level not changed, though)
• Updated HowToBuild to remind that additional local.properties files needed.
• Added the additional Android support library, for the 3rd party package of drag-sort-listview

[geometer]

Thanks. The correct solution for the issues is to switch to gradle; and move org.fbreader.util to a separate module. This is already done for fbreader premium, and will be backported to this repo after 2.6 release.

[goldyliang]

Support for moving to gradle. It will be much more convenient for continurous development in gradle and android studio.