Reduce a snprintf() call in localjump_error().
Replace tabs with spaces
Move src/sprintf to mrbgems/mruby-sprintf/src/
Merge pull request #942 from kano4/master

Replace tabs with spaces
Separate Kernel#sprintf support from mruby core. It's moved to mrbgems.
Merge pull request #943 from monaka/pr-tmp-move-sprintf-to-mrbgem

Move Kernel#sprintf to mrbgem
save/restore arena index for each codedump iteration
proper irep pool duplication check for strings
Added a link to the contribution guidelines in the README
Merge pull request #945 from carsonmcdonald/refguidelines

Added a link to the contribution guidelines in the README
Merge pull request #937 from monaka/pr-remove-snprintf-from-localjump_error

Reduce a snprintf() call in localjump_error().
use mrb_str_buf_new to create a buffer to be used by mrb_str_buf_cat