Merge pull request #1106 from monaka/pr-remove-mrb_string_value

Remove API mrb_string_value().
Fixed testrb load path
Merge pull request #1107 from masuidrive/fixed_testrb_path

Fixed testrb load path
Remove mrb_flt2big() as there is no bignum in the core.
 Add new API mrb_flo_to_fixnum(). You can replace mrb_flt2big() to mrb_flo_to_fixnum() with few modifications.
Change the second argument of mrb_flo_to_str().
 Export mrb_flo_to_str() as API.
Include mrbconf.h as MRB_INT_MAX used here.
Rename mrb_fix2str() to mrb_fixnum_to_str(). This is for naming consistency.