[SPARK-4562] [MLlib] speedup vector

This PR change the underline array of DenseVector to numpy.ndarray to avoid the conversion, because most of the users will using numpy.array.

It also improve the serialization of DenseVector.

Before this change:

trial	| trainingTime | 	testTime
-------|--------|--------
0	| 5.126 | 	1.786
1	|2.698	|1.693

After the change:

trial	| trainingTime |	testTime
-------|--------|--------
0	|4.692	|0.554
1	|2.307	|0.525

This could partially fix the performance regression during test.

Author: Davies Liu <davies@databricks.com>

Closes #3420 from davies/ser2 and squashes the following commits:

0e1e6f3 [Davies Liu] fix tests
426f5db [Davies Liu] impove toArray()
44707ec [Davies Liu] add name for ISO-8859-1
fa7d791 [Davies Liu] address comments
1cfb137 [Davies Liu] handle zero sparse vector
2548ee2 [Davies Liu] fix tests
9e6389d [Davies Liu] bugfix
470f702 [Davies Liu] speed up DenseMatrix
f0d3c40 [Davies Liu] speedup SparseVector
ef6ce70 [Davies Liu] speed up dense vector

(cherry picked from commit b660de7a9cbdea3df4a37fbcf60c1c33c71782b8)
Signed-off-by: Xiangrui Meng <meng@databricks.com>
get raw vectors for further processing in Word2Vec

e.g. clustering

Author: tkaessmann <tobias.kaessmann@s24.com>

Closes #3309 from tkaessmann/branch-1.2 and squashes the following commits:

e3a3142 [tkaessmann] changes the comment for getVectors
58d3d83 [tkaessmann] removes sign from comment
a5be213 [tkaessmann] fixes getVectors to fit code guidelines
3782fa9 [tkaessmann] get raw vectors for further processing
[SPARK-4578] fix asDict() with nested Row()

The Row object is created on the fly once the field is accessed, so we should access them by getattr() in asDict(0

Author: Davies Liu <davies@databricks.com>

Closes #3434 from davies/fix_asDict and squashes the following commits:

b20f1e7 [Davies Liu] fix asDict() with nested Row()

(cherry picked from commit 050616b408c60eae02256913ceb645912dbff62e)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>