BLD: also clean pyo files
DOC: put contribution guidelines in CONTRIBUTING.md
DOC: show travis build status on github page
Update doc/source/io.rst

Tiny docs fix.
Merge pull request #2774 from y-p/fix_guidelines

add travis badge to GH page, imporve contributor guildelines
DOC: update CONTRIBUTING.md
BLD: Travis - only install numpy when we have to, and use 1.7rc1 for 3.3
BLD: Travis workaround for latest breakage

recent image deploy on travis ignores "system_site_packages"
in travis.yml. Implement workaround. yet again.
Merge branch 'master' into squeeze