move all fortran files into main directory

rename module lbfgsb -> lbfgsb_entry (avoid name conflict when compil…

…ing with cgo)

rename lbfgsb_entry -> lbfgsb__entry (compile order)

lbfgsb -> lbfgsb_entry in module description

rename lbfgsb.f90 to avoid conflicts

using new go modules + minor updates to example & gitignore

Updated README for initial release.