Updated gradle build files.

Updated gradle build files so that ActionBarSherlock can be
correctly referenced as a subproject in another project according
to Xavier Ducrohet's recommendation:
http://stackoverflow.com/a/16974411

1. Each gradle.build file now contains the buildscript definition
and classpath dependency on the android gradle plugin.

2. Updated settings.gradle project file so that each subproject's
projectDir is defined as suggested by Xavier.