Release 0.2.0

[pwittchen]

Initial release notes:

• added WifiState enum
• added Observable<WifiState> observeWifiStateChange(context) to ReactiveWifi class
• updated Gradle and Travis CI config
• updated Gradle Wrapper version
• bumped target SDK version to 25 and build tools version to 25.0.1
• bumped RxJava to v. 1.2.6
• made methods, which create Observables in ReactiveWifi class static
• made the constructor of ReactiveWifi class private
• added permission annotations
• added AccessRequester class responsible for checking if Location Services are enabled and redirecting the user to Location Settings

Things to do:

• bump library version to 0.2.0
• upload Archives to Maven Central Repository
• close and release artifact on Nexus
• update JavaDoc on gh-pages
• update CHANGELOG.md after Maven Sync
• update download section in README.md after Maven Sync
• create new GitHub release