[Spark-6693][MLlib]add tostring with max lines and width for matrix

jira: https://issues.apache.org/jira/browse/SPARK-6693

It's kind of annoying when debugging and found you cannot print out the matrix as you want.

original toString of Matrix only print like following,
0.17810102596909183    0.5616906241468385    ... (10 total)
0.9692861997823815     0.015558159784155756  ...
0.8513015122819192     0.031523763918528847  ...
0.5396875653953941     0.3267864552779176    ...

The   def toString(maxLines : Int, maxWidth : Int) is useful when debuging, logging and saving matrix to files.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #5344 from hhbyyh/addToString and squashes the following commits:

19a6836 [Yuhao Yang] remove extra line
6314b21 [Yuhao Yang] add exclude
736c324 [Yuhao Yang] add ut and exclude
420da39 [Yuhao Yang] Merge remote-tracking branch 'upstream/master' into addToString
c22f352 [Yuhao Yang] style change
64a9e0f [Yuhao Yang] add specific to string to matrix