Merge pull request Angel-ML#942 from Gscim/branch-3.1.0

Branch 3.1.0

remove some constants

Merge pull request Angel-ML#847 from rachelsunrh/develop

fix link

optimize line doc

[DOCS] fix link

Merge branch 'master' into branch-2.1.0

Merge branch 'hotfix' into branch-2.0.2

# Conflicts:
#	angel-ps/mllib/src/test/java/com/tencent/angel/ml/logisticregression/LRTest.java

[DOCS] remove "online" and "offline" title as "FTRL" can be use onlin…

…e or offline

[DOCS] fix doc format

change spark version