evaluator for recurrent group.

Using previous image for travis-ci

Fix bugs for rnn generation.

Because travis-ci has been updated Ubuntu Trusty image, it causes
Paddle CI building error. Just using old image now for hot-fix, I
will add another issue to fix Paddle building in new TravisCI image.

Related link
https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch

Handle multiple processes trying to create the data home directory

Add an order switch to xmap_readers

Rewrite tutorial comments in generic.cmake

ENH: Merge multiple static libs into singe one

Fix unit test error: File exists: '/root/.cache/paddle/dataset'

FIX: Add boost inc dir

Add WITH_GOLANG to control the link of go lib

Allow printer layer to print user-provided message

Move paddle/majel/* to paddle/{platform,framework/*

Make the evaluators inside a recurrent goup true evaluator, meaning that their evaluation results are incorporated into the whole evaluator result.

1. v2.layer.parse_network does not correctly handle the generation output.
2. GatherAgentLayer does not correctly handle generation output when batch_size > 1
3. Fix CustomStackTrace for rnn group