add mrb_str_to_cstr
Merge pull request #836 from iij/pr-mrb_str_to_cstr

string contains null byte check utility function (mrb_str_to_cstr)
Update doc/compile/README.md
Added String#sub/sub! and String#gsub/gsub!
Merge pull request #838 from brainopia/patch-1

Fix typos in compilation guide
Merge pull request #839 from masuidrive/string_sub_gsub

Added String#sub/sub! and String#gsub/gsub!
Extend MiniRake::DSL module to prevent inheritance pollution

```include MiniRake::DSL``` on toplevel will pollute ancestor tree of every object.

```extend MiniRake::DSL``` will add those methods only to ```main``` object.

Rake does the same: https://github.com/jimweirich/rake/blob/master/lib/rake/dsl_definition.rb#L153
add Module#remove_const
use mrb_get_args("n") instead of decomposing each time
Merge pull request #840 from brainopia/patch-2

Extend MiniRake::DSL module to prevent inheritance pollution
Merge branch 'master' of github.com:mruby/mruby
fix error output for Module#remove_const
Add Module#remove_const test
String#gsub fix with last character
Fix crash when exiting mirb using ctrl-d when compiled with readline
support.
Merge pull request #844 from skandhas/pr-fix-error-output-for-remove_const

fix incorrect output for Module#remove_const
Merge pull request #846 from bovi/remove_const_doc

Add Module#remove_const Test Case
Merge pull request #847 from bovi/gsub

String#gsub fix with last character
Merge pull request #848 from carsonmcdonald/mirbreadlinefix

Fix mirb crash
:Merge branch 'master' of github.com:mruby/mruby

Conflicts:
	src/class.c