Update README.md
add build-tool v19.1 update README and box version
Merge pull request #4 from fly1tkg/master

add build-tool v19.1
update Android SDK version
change android-19 system image name
add validate-wercker-box step
add all option to android sdk update

Conflicts:
	update-sdk.sh
	wercker-box.yml
update gradle version to 1.12
update README and box version
Merge pull request #5 from fly1tkg/new-sdk

Updated Android SDK to 23, gradle to 1.12
Merge with master and add support for SDK version 23.0.2