Maven Central/Sonatype configuration

Adding configuration for uploading to Sonatype for syncing artifacts to Maven Central.

For the `uploadArchives` task to work you need to:

1) Have an account with access to the `com.simperium` group on https://issues.sonatype.org/secure/Dashboard.jspa
2) Configure the username and password for that account in `gradle.properties`
3) Setup a GPG key and configure the correct `gradle.properties`

```
signing.keyId=PRIVATE_KEY
signing.password=PRIVATE_KEY_PASSWORD
signing.secretKeyRingFile=PATH_TO_GPG_KEYRING

ossrhUsername=SONATYPE_USERNAME
ossrhPassword=SONATYPE_PASSWORD
```