global rename Activity --> Task. Java part compiles and runs unit tests; typos not fixed yet (e.g. "an task" in the comments)
replaced "an task" in the comments
more "an task" fixes
more renaming fixes - mostly in the comments; Activities -> Tasks (plural)
Merge branch 'master' of github.com:Microsoft-CISL/REEF into sergiym_rename_task

Conflicts:
	reef-common/src/main/java/com/microsoft/reef/runtime/common/driver/EvaluatorManager.java
Merge branch 'master' of github.com:Microsoft-CISL/REEF into sergiym_rename_task

Conflicts:
	reef-examples/src/main/java/com/microsoft/reef/examples/DBCModel/JobClient.java
	reef-examples/src/main/java/com/microsoft/reef/examples/DBCModel/JobDriver.java
Merge branch 'master' of github.com:Microsoft-CISL/REEF into sergiym_rename_task

Conflicts:
	reef-io/src/main/java/com/microsoft/reef/io/network/group/impl/operators/basic/config/GroupCommOperators.java
	reef-io/src/main/java/com/microsoft/reef/io/network/impl/NetworkService.java
	reef-io/src/main/java/com/microsoft/reef/io/network/util/Utils.java
	reef-io/src/test/java/com/microsoft/reef/services/network/NetworkServiceTest.java
more renames activity -> task in group comm and around
Removed some more instances of "Activity"
Merge pull request #678 from Microsoft-CISL/sergiym_rename_task

Rename Activity into Task