Changed Vanadium in modified abundances file for split_nuclide script.
Added derivation for reflection off of cones in documentation.
Ability to specify natural elements in materials. Closes #97.
Fixed reaction_name for MT=849.
Added test for <element>.
Added test for multiple S(a,b) tables.
Added description of <element> to user's guide.
Updated release notes.
Changed protocol for git clone in install instructions.
Add checks for redundant unique IDs in user input.
Added cross_sections.xml for MCNP-distributed ASCII cross sections.
Added save attribute in input_xml module.
Added Blue Gene/Q compilation option.
merged master