Adds plugin and details to upload to bintray

Uses this plugin https://github.com/novoda/bintray-release to make the library releaseable.

(after bintray website setup)
You can release with this command:

`./gradlew clean build bintrayUpload -PdryRun=false -PbintrayUser=BINTRAY_USERNAME -PbintrayKey=BINTRAY_KEY `

I just added the minimum config to the release details, but you can add a lot more if you want: https://github.com/novoda/bintray-release/wiki/Configuration-of-the-publish-closure

Obviously the details are your choice, I just picked some sane defaults:

```groovy
    userOrg = 'adw'
    groupId = 'org.adw.library'
    artifactId = 'discreteseekbar'
```

Before you can push your release to BinTray you need to sign up by following these steps, which are straight forward enough: https://bintray.com/howbintrayworks#page11

You can test your release with `-PdryRun=true` and it will create the files but not push them up the BinTray, if you are interested under the hood it performs the same actions as this http://blog.blundell-apps.com/locally-release-an-android-library-for-jcenter-or-maven-central-inclusion/