Add `idtype` for eutils queries

[cjfields]

Recent changes at NCBI will essentially obsolesce GI in favor of accession.version. A new idtype parameter can be used with eutils to allow submitted and returned IDs to be accessions opposed to versions. We can add this (the change won't go live until late Sept 2016 but the option will simply be ignored). Note: the removal of GI won't happen until mid-Oct 2016.

[cjfields]

See this YouTube video for specifics.