DataFrame.from_items clean-up

- Improve docstrings and error messages
- Convert final `elif` to `else`
Update doc/source/install.rst

Python 2.5 isn't supported
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.
DOC: Fix syntax so bug listing renders properly
DOC: improving docstring for swaplevel method #2648
Merge pull request #2757 from dandavison/dataframe_from_items_cleanup

DOC: DataFrame.from_items clean-up
ENH: add link to travis build status image
Fix typo in performance_doc
DOC: fix typo in CONTRIBUTING.md
Merge pull request #2820 from stephenwlin/patch-1

DOC: fix typo in CONTRIBUTING.md
Merge pull request #2812 from rgieseke/patch-2

Fix typo in performance_doc
Merge pull request #2762 from jamescasbon/patch-1

Update doc/source/install.rst
DOC: fix corr() docstring

thanks @rgieseke #2399