Snowplow Android Tracker v0.2.0

Upgraded to take advantage of Core v0.2.0; various improvements to development and build process

Bug fixes

• Bumped Core version to 0.2.0 (#42)
• Updated CHANGELOG with missing tickets (#41)
• Added GET and POST logging in the Emitter, thanks @lixiaoyi! (#38)

Build process

• Updated to latest gradle tools and plugin, thanks @hamidp! (#39)
• Targeted latest SDK version (21) (#47)
• Fixed Gradle Dynamic Version linting warning (#46)

Development environment

• Added dedicated Vagrant setup (#40)
• Added warning that Version.java is auto-generated (#45)
• Added Quickstart section to README (#43)