updated pyne for new version of xdress
Adding tested geometry discretization capability.
PEP 8
More style fixes.
Cell volume fractions to mixed materials mesh. Method and tests.
PEP8
Docstring clarification.
Masking and other cleanup for PR.
Minor docstring fix.
Merge pull request #346 from elliottbiondo/cell_mixing

Geometry cell material mixing
Merge pull request #343 from scopatz/xd

Updated pyne for new version of xdress
Adding Xsdir class documentation.
Xsdir and XsdirTable class tests
Fixed typo.
Merge pull request #348 from elliottbiondo/xsdir_tests

Xsdir tests
Merge branch 'staging' of https://github.com/pyne/pyne into q_vals

updating q_vals with pyne changes