Fix typos in README

* Corrected a couple small grammar and puncuation errors.
* Fixed missing semicolon & curly brace
* Removed reference to `Path` interface
* Update to new android gradle plugin name
* Update android gradle plugin version to 0.13.+
* Switch to jcenter() from mavenCentral()
  * https://code.google.com/p/android/issues/detail?id=72061