fix handling of :branch option of gem.conf

add mrbgems/mruby-string-ext, and method: String#getbyte

Add Support to build loadable modules defined in mrbgems.

Remove warning for regexp

Add null check after mrb_realloc for array

Add include guard in node.h and Replace tabs with spaces

Skip end of loop in mrb_sym2name_len

Remove RB_GC_GUARD(x) macro.

Cleanup stdio related.

Add accessor: version, description, summary, homepage.

Reduce a snprintf() call in localjump_error().

  C API mrb_p() is always callable.
  - But it will do nothing when ENABLE_STDIO is not defined.

  Kernel#__printstr__ is always callable.
  - But it will do nothing when ENABLE_STDIO is not defined.

Added a link to the contribution guidelines in the README

Move Kernel#sprintf to mrbgem

Replace tabs with spaces