mrdb, mirb: Add fflush() so that a external program can read output imme...

Don't use Build.current if possible

mrdb: fix crash when stepping into Proc.call

update stable version number in README

How to reproduce:

```
cat a.rb
Proc.new { 1 }.call

echo step | mrdb a.rb
```

fix infinite loop in String#match(arg) when arg is String

Add String#upto

add String#ljust into mruby-string-ext

fix Segmentation fault on Struct#inspect due to recursive Struct object

Use sizeof() instead of strlen().

Properly implement directory tasks

Use mrb_exc_new_str_lit().

Removed duplicated declarations.

 * `mrb_show_version()`
 * `mrb_show_copyright()`

fixed error-handling for mrb_open().

When mrb_open() is called again, it is not checked.

Round execution time

Set correct build dir for bins

Remove redundant NULL checks for mrb_malloc().

Avoid block_given? in mrbgem:enum.rb to reduce method calls

Use suitable type.

mruby-eval: fix typo