Updated path to ABS library and added clarification of directory location

ABS library directory was moved from library to actionbarsherlock, per commit from @JakeWharton https://github.com/JakeWharton/ActionBarSherlock/commit/2c40e6fa1846c2af73d48ea1760387e0aea07e12

Added clarification of the directory location where the commands are executed from

Build-target has to be at least 13 or ABS dependencies will cause the build to fail

ABS branch 4.2.0 doesn't exist

Great document and was very helpful - minor corrections overall to https://github.com/WhisperSystems/TextSecure/pull/208