Improve Grammar of Documentation

Implement gembox-relative gemdir paths

In the case where a relative path is specified to a gembox from
build_config.rb, it was previously tricky to specify relative gem paths
from inside that gembox.

For example, consider a project in which mruby is checked out as a
submodule in the project root:

	+- project_root
	   |
	   +- mruby/
	   |  |
	   |  +- build_config.rb
	   |  |
	   |  +- ...
	   |
	   +- my_gembox/
	      |
	      +- my_gembox.gembox
	      |
	      +- my_gem/
	         |
	         +- mrbgem.rake
	         |
	         +- ...

If build_config.rb refers to my_gembox with a relative path, it's
difficult for my_gembox to then refer to my_gem. With this proposed
change, my_gembox.gembox can look like this:

	MRuby::GemBox.new do |conf|
	  conf.gem "my_gem"
	end

Added test for inline arrays longer than 126 elements.

fix #1519

the 128th element in an array literal would trigger a corner case on
splat mode checking, in which mruby will splat an that value into its
parent array. the issue was masked by the fact of ary_concat() also
accept non-array value:

1.9.3p286 :002> a = 1
1.9.3p286 :003> [*a]
 => [1]

the expected behaviour should be OP_ARYPUSH the 128th element, instead of
splat it by OP_ARYCAT.

Remove function delaration not implemented in code

fix build error when ENABLE_DEBUG is defined

fix bugs on Heredocument

eliminate use of traditional intern API (mrb_intern()) completely

- heredoc in array literal
- heredoc in args
- heredoc in expression expand

Use self for the given proc in mrb_yield

Fixed self value in a block is changed with return value

remove executable bit from include/mruby/dump.h file

Fix #1504

fix #1504

Fix checking return value of fread()

Fix insufficient preparation to call mrb_debug_info_append_file.