Make user can check what type is in variable

Adding platform/must_check.h

__must_check is a macro mark of function return value. It let developer
must check the return value is legal or not.

Implement framework::Variable

Fix macos compile

Please use `override` not `virtual` in sub-classes.
`override` can check if there is a method in `parent` while compiling.

Master save and load state from etcd, and register on etcd.

fix etcd lease

Fix the problem that protobuf cannot be used as a DEPS argument in cc…

Improve pruning  module

Simplify Travis CI configuration

"resolve clock skewed"

add coeff parameter to classification_cost

Add DetectionMAPEvaluator

I made a comment in WuYi's PR that this is not necessary, so WuYi
removed it. Turns out it's necessary after confirming with coreOS
developer.

Pserver etcd registration

evaluator for recurrent group.

Using previous image for travis-ci