Custom Builds

This change will let us have custom builds for everyone! Jar, Gradle, Maven or even library project users. 

We check if the integration is bundled on the device. If we don't have it on our device, we'll simply send them to our REST API, which takes care of sending it to the right integration. Otherwise, we disable sending any payloads to that integration from our servers, and call the bundled integration (the native libraries) instead.

### Default Build
There will be a `analytics-android-all` artifact that will include all our bundled integrations. We'll show this in our Quickstart guides. NOTE : This is only available for build system users currently. We'll roll out support for Jar users, but not currently.

```groovy
dependencies {
  compile 'com.segment.android:analytics-android-all:+'
}
```

### Minimal Build
For users that don't want all integrations, they can use our `analytics-core` artifact. JAR users can simply drop `analytics-android-core.jar`, or add it as a dependency. All events will be sent through our servers.
```groovy
dependencies {
  compile 'com.segment.android:analytics-core:+'
}
```

### Custom Build
For users that don't want all the integrations, but still want to take advantage of the native features of specific integrations (e.g. sessions), they can build upon our `analytics-core` artifact.

For a service that needs Mixpanel and Localytics, just add the required artifacts as a dependency.
```groovy
dependencies {
  compile 'com.segment.android:analytics-core:+'
  compile 'com.segment.android:analytics-localytics:+'
  compile 'com.mixpanel.android:mixpanel-android:4.2.1@aar'    
}
```

###### JAR Users
JAR users will add `analytics-core.jar`, `analytics-localytics.jar` and the native library for Mixpanel.