support freebsd versions without log2 function
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
Fix for caller_dir relative gems
Merge pull request #1523 from wrl/relative-gembox-nokeyword

Implement gembox-relative gemdir paths
Merge pull request #1522 from bovi/patch-1

Improve Grammar of Documentation
fix no multiline strings in mirb
Merge pull request #1528 from Ahti/master

fix no multiline strings in mirb
Disable implicit integer conversion in mrb_get_args
simplify stack_clear()
add some methods to Symbol
remove unused value for mrb_gv_remove
add mrb_gv_remove(); untested
Merge pull request #1535 from matsumoto-r/del_unused_varaiable

remove unused value for mrb_gv_remove
Merge pull request #1534 from skandhas/pr-add-some-symbol-methods

Add some methods to Symbol
define Module#const_missing
better error position display
Set regs to stack on return of const_get calls before use
Merge pull request #1538 from carsonmcdonald/conststackfix

Set regs to stack on return of const_get calls before use
move declarations to the beginning of blocks