Merge pull request #161 from gmatteo/master

print_function
fix bug in bond_valence
generalize SymmOp.operate_multi to work on higher dimensional arrays
line_density ->kpoints_line_density
Merge pull request #162 from gmatteo/master

add atomicfile to install_requires
added Structure.merge_sites
Merge branch 'master' of github.com:materialsproject/pymatgen