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: 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.
BUG: DataFrame.clip arguments were inconsistent to numpy and Series.clip, closes GH #2747
BUG: various bug fixes for DataFrame/Series construction related to:
        0 and 1 len ndarrays
        datetimes that are single objects
        mixed datetimes and objects (GH #2751)
        astype now converts correctly with a datetime64 type to object, NaT are converted to np.nan
        _get_numeric_data with empty mixed-type returning empty, but index was missing
DOC: release notes updated, added missing_data section to docs, whatsnew 0.10.2
BUG: issue in PyTables with too many selectors in a where
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
TST: use ensure_clean contextmanager to avoid spewing temporary files if
     testing is interrupted
DOC: added DataTypes section to HDFStore
ENH: provide dotted (attribute) access in stores (e.g. store.df == store['df'])