added test to check natural_abund
Merge remote-tracking branch 'upstream/staging' into staging
added orig sphinx ext
made some doc changes
Added a abundance_by_Z function to data.pyx
Merge remote-tracking branch 'upstream/staging' into staging
changed abundance_by_Z to module-level dict
removes import guards from mcnp.py and adds them to tests, skipping tests if include guards are hit. closes #158.
removed commented out sections
fixed backgrounds
made abundance_by_Z interval open-open, added test to test for soundness of abundance values
Merge branch 'staging' of github.com:pyne/pyne into staging
Merge pull request #166 from scopatz/docfix

Docfix
updated abundance test
Merge branch 'abund' into staging