[SPARK-5424][MLLIB] make the new ALS impl take generic ID types

This PR makes the ALS implementation take generic ID types, e.g., Long and String, and expose it as a developer API.

TODO:
- [x] make sure that specialization works (validated in profiler)

srowen You may like this change:) I hit a Scala compiler bug with specialization. It compiles now but users and items must have the same type. I'm going to check whether specialization really works.

Author: Xiangrui Meng <meng@databricks.com>

Closes #4281 from mengxr/generic-als and squashes the following commits:

96072c3 [Xiangrui Meng] merge master
135f741 [Xiangrui Meng] minor update
c2db5e5 [Xiangrui Meng] make test pass
86588e1 [Xiangrui Meng] use a single ID type for both users and items
74f1f73 [Xiangrui Meng] compile but runtime error at test
e36469a [Xiangrui Meng] add classtags and make it compile
7a5aeb3 [Xiangrui Meng] UserType -> User, ItemType -> Item
c8ee0bc [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into generic-als
72b5006 [Xiangrui Meng] remove generic from pipeline interface
8bbaea0 [Xiangrui Meng] make ALS take generic IDs