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
Merge branch 'master' into iij

Conflicts:
	mrblib/string.rb
	test/t/string.rb
add Module#remove_const
Merge pull request #840 from brainopia/patch-2

Extend MiniRake::DSL module to prevent inheritance pollution
Merge branch 'master' into iij
add assert_{true,nil,equal} methods.
File.exists? may fail for shared string
Merge branch 'pr-mrbtest-assert-ext' into iij

Conflicts:
	test/assert.rb
add assert_{includes,instance_of,kind_of,raises} methods.
remove assert_* test
Regexp: %r notation bug fix. close #85.

be able to parse any delimiters.
variable expansion isn't supported yet.

 * sample code
```
def matchpath(str)
  puts "#{str} is ..."

  if %r!img/! =~ str then
    puts "match"
  else
    puts "not match"
  end
end

matchpath("img/a.png") # => match
matchpath("/usr/local/bin") # => not match
matchpath("book/img/local") # => match
```
add tests for Regexp#source and Regexp#to_s.
add Integer#chr method
add String#getbyte

Conflicts:

	src/string.c
	test/t/string.rb