Added a link to the contribution guidelines in the README

Move Kernel#sprintf to mrbgem

Replace tabs with spaces

Use MRB_IREP_ARRAY_INIT_SIZE macro. 

Define ARY_MAX_SIZE strictly. Use mrb_int instead of int.

Clean up range.c. Use mrb_int. Use mrb_vtype.

Remove unused var len

mrb_open takes no arguments: (void) instead of ()

cleanup: delete commented out match_op() from parse.y

Adjust some indents

cleanup: unused variable local_svar

Remove trailing whitespaces. This is just a cosmetic change.

bugfix: assert_raise

Remove unused var sv