Move directory third_party into /build

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.