gradelization Multi-Project + Maven upload

Hi,

I've gradelized the project:

What's there:
- Gradle wrapper
- Multi-project, including the samples (one-click launch from android studio)
- Maven pom/aar upload (see at the end for how to configure this)

What's not there:
- Launch tests from gradle. I can't get the facebook/tests project to work with gradle. Ant and the build_and_test.sh script still work.

A Word about maven: you need to add this
```bash
mavenRepoSnapshot=YOUR_MAVEN_SNAPSHOT_REPO
mavenRepoRelease=YOUR_MAVEN_RELEASE_REPO
mavenUsername=YOUR_MAVEN_UPLOAD_USER_NAME
mavenPassword=YOUR_MAVEN_UPLOAD_PASSWORD
```
in `~/.gradle/gradle.properties`

You can use your own repo but it would be great if the library was in Maven Central. You'll have to sign up for Sonatype and get signing keys. The line that signs the archive is commented for now but when you have keys and add
```bash
signing.keyId=KEY
signing.password=PASSWORD
signing.secretKeyRingFile=GPG_FILE
```
in `~/.gradle/gradle.properties` you can uncomment it.

You'll see that I had to remove the IDEA files as there was lots of changes. IDEA Gradle integration works well. I haven't touched eclipse and ant.

Thanks for considering

--Gilles