Add Support to build loadable modules defined in mrbgems.
Apply @masuidrive's review point. @bins should be Array.
fix handling of :branch option of gem.conf
add mrbgems/mruby-string-ext, and method: String#getbyte
Merge pull request #961 from monaka/pr-support-user-defined-loadable-build

Add Support to build loadable modules defined in mrbgems.
Merge pull request #963 from skandhas/pr-add-mrbgems-mruby-string-ext

add mrbgems/mruby-string-ext, and method: String#getbyte
Merge pull request #962 from masahino/fix_branch_option

fix handling of :branch option of gem.conf
unify mrb_ary_new{,_elts,_from_values}; mrb_ary_from_values() is a winner