[SPARK-1580][MLLIB] Estimate ALS communication and computation costs.

Continue the work from #493.

Closes #493 and Closes #593

Author: Tor Myklebust <tmyklebu@gmail.com>
Author: Xiangrui Meng <meng@databricks.com>

Closes #1731 from mengxr/tmyklebu-alscost and squashes the following commits:

9b56a8b [Xiangrui Meng] updated API and added a simple test
68a3229 [Xiangrui Meng] merge master
217bd1d [Tor Myklebust] Documentation and choleskies -> subproblems.
8cbb718 [Tor Myklebust] Braces get spaces.
0455cd4 [Tor Myklebust] Parens for collectAsMap.
2b2febe [Tor Myklebust] Use `makeLinkRDDs` when estimating costs.
2ab7a5d [Tor Myklebust] Reindent estimateCost's declaration and make it return Seqs.
8b21e6d [Tor Myklebust] Fix overlong lines.
8cbebf1 [Tor Myklebust] Rename and clean up the return format of cost estimator.
6615ed5 [Tor Myklebust] It's more useful to give per-partition estimates.  Do that.
5530678 [Tor Myklebust] Merge branch 'master' of https://github.com/apache/spark into alscost
6c31324 [Tor Myklebust] Make it actually build...
a1184d1 [Tor Myklebust] Mark ALS.evaluatePartitioner DeveloperApi.
657a71b [Tor Myklebust] Simple-minded estimates of computation and communication costs in ALS.
dcf583a [Tor Myklebust] Remove the partitioner member variable; instead, thread that needle everywhere it needs to go.
23d6f91 [Tor Myklebust] Stop making the partitioner configurable.
495784f [Tor Myklebust] Merge branch 'master' of https://github.com/apache/spark
674933a [Tor Myklebust] Fix style.
40edc23 [Tor Myklebust] Fix missing space.
f841345 [Tor Myklebust] Fix daft bug creating 'pairs', also for -> foreach.
5ec9e6c [Tor Myklebust] Clean a couple of things up using 'map'.
36a0f43 [Tor Myklebust] Make the partitioner private.
d872b09 [Tor Myklebust] Add negative id ALS test.
df27697 [Tor Myklebust] Support custom partitioners.  Currently we use the same partitioner for users and products.
c90b6d8 [Tor Myklebust] Scramble user and product ids before bucketing.
c774d7d [Tor Myklebust] Make the partitioner a member variable and use it instead of modding directly.