[SPARK-2848] Shade Guava in uber-jars.

For further discussion, please check the JIRA entry.

This change moves Guava classes to a different package so that they don't conflict with the user-provided Guava (or the Hadoop-provided one). Since one class (Optional) was exposed through Spark's public API, that class was forked from Guava at the current dependency version (14.0.1) so that it can be kept going forward (until the API is cleaned).

Since sbt doesn't provide the relocation feature maven-shade-plugin does, I implemented the functionality (based on the souce for maven's plugin).

Note this change has a few implications:
- *all* classes in the final jars will reference the relocated classes. If Hadoop classes are included (i.e. "-Phadoop-provided" is not activated), those will also reference the Guava 14 classes (instead of the Guava 11 classes from the Hadoop classpath).
- if the Guava version in Spark is ever changed, the new Guava will still reference the forked Optional class; this may or may not be a problem, but in the long term it's better to think about removing Optional from the public API.

For the end user, there are two visible implications:

- Guava is not provided as a transitive dependency anymore (since it's "provided" in Spark)
- At runtime, unless they provide their own, they'll either have no Guava or Hadoop's version of Guava (11), depending on how they set up their classpath.
If you leave it as compile, it will pollute the compilation classpath of people using Spark in their projects (Guava will be pulled as a compile-scope dependency too).

http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Dependency_Scope

Adding it as compile-scope in the assembly project is then needed because maven-shade-plugin will only include those in the final jar (pretty sure there are setting to include other scopes, but didn't want to play with those).