This fixes #6: adds ability to publish library to Maven Central.

This prepares the build.gradle file for publishing the library to Maven Central.
It's in the lines of this article here:  http://yennicktrevels.com/blog/2013/10/11/automated-gradle-project-deployment-to-sonatype-oss-repository/

Next steps:
- create an account with OSS Sonatype for the "com.sendgrid" group
(see https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide)

- create a public key and configure your ~/.gradle/gradle.properties
(see https://docs.sonatype.org/display/Repository/How+To+Generate+PGP+Signatures+With+Maven and  http://yennicktrevels.com/blog/2013/10/11/automated-gradle-project-deployment-to-sonatype-oss-repository/ )

- do your first publish to Maven Central
```
$ gradle uploadArchives
```
and release the artifacts: https://oss.sonatype.org/index.html#stagingRepositories