added a sub_elem function which returns a material of a given elemental slice. replaced implementation of sub_u and sub_pu with specific instance of sub_elem. added tests for sub_mat and set_mat.
fixed documentation of set_mat and sub_mat functions.
updated material constructor to also pass ints in a nucvec through the nucname.id function. now materials constructed with {92:1} will have a comp of {92000000:1}. these three commits close #131.
Merge branch 'i131' of github.com:gidden/pyne into gidden-i131
fix for serpent tests