First class support for --packages

A lot of people are using --packages to include packages like spark.csv.

Right now users and manaully specify spark.jars.packages, spark.jars.ivy and spark.jars.excludes in conf in the session request.

But it might be easier for users if we provide a first class options in in the session request like:

{
    packges: ["com.databricks:spark-csv_2.10:1.4.0"]
    excludePackages: ["org.apache.hadoop:*"]
}