Added Unity Plugin.

Files:
- Assets/Plugins/TapSense.cs: Exposes SDK methods to Unity (C# and JavaScript)
- Assets/Plugins/TapSensePrefab.prefab: Enables callbacks when placed in a scene
- Wrappers around TapSenseAds SDK
  - Assets/Plugins/Android/ts-android-pub-sdk.jar (includes Unity Bridge file)
  - Assets/Plugins/iOS/TapSense.h and .m
- Assets/Plugins/Android/AndroidManifest.xml: Adds permissions and activities to the default Unity AndroidManifest.xml
- TapSenseAds.unitypackage
- Sample App - modified AngryBots to demo the Unity Plugin:
  * Made pause buttons and menu buttons bigger
  * Clicking on pause shows an interstitial and banner
  * Clicking on pause again hides the banner

All public APIs are exposed with callbacks, with the following differences:
- interstitial.requestAd() and interstitial.showAd() don't return booleans
- Added BannerPosition and AdSize to banner constructor
- Added banner.setVisibility(bool)

Compatible with TapSense Ads iOS SDK v2.1.6+ and TapSense Ads Android SDK v2.1.6+.

Updated .gitignore for Unity files