Merge tag 'shifu-0.2.2' of github.com:ShifuML/shifu

[maven-release-plugin]  copy for tag shifu-0.2.2
Updated travis-ci before_script to clone and install guagua
Updated README.md
Updated README.md
Updated README.md
Updated README with travis build, and updated travis.yml file
Updated travis.yml file
Updated travis.yml file, changed from before_script to before_install
Disable guagua tests in travis builds
Fix README formatting
Update README.md
fix unit-tests that maybe fail sometimes; update .gitignore
remove duplicate data filer in Normalize.pig
remove duplicate sampling logic in normalization and rename DataFilter to DataSampler
Fix unittest
change default normalization samplerate as 1.0
filterExpressions doesn't work if the field is null
Merge branch 'master' into develop
Merge branch 'develop' of https://github.com/ShifuML/shifu into develop
Merge branch 'develop' of github.com:ShifuML/shifu into develop