Fix #545: Update build.gradle to make it work with gradle-android 0.7.+

* Remove unused libs/*.jar files (fetched from the maven repository)
* Update gradle wrapper to gradle 1.9
exclude some files from packaging to avoid duplicates in final apk
Merge pull request #550 from wordpress-mobile/issue/545-update-gradle-android-0.7

Fix #545 update gradle android 0.7