fix #1550

fixed compilation on freebsd releases witout log2 function

emit a "\n" as the first token for parser instead of taking the
first character from the next file for lexer, to prevent mruby
"concatenate" two keywords between files

it should be regarded as a work around, for it can not resolve
this case:

$ cat a.rb
"
$ cat b.rb
b"

$ bin/mrbc -o- -v a.rb b.rb
mruby - Embeddable Ruby  Copyright (c) 2010-2013 mruby
developers
NODE_SCOPE:
        NODE_BEGIN:
                NODE_STR "

b " len 4
irep 0 nregs=2 nlocals=1 pools=1 syms=0
000 OP_STRING   R1      "\n\nb "
001 OP_STOP

thanks @bovi 's idea

Disable implicit integer conversion in mrb_get_args

We have already reserved that register in codegen()'s case NODE_DEF, but
what we care about is actually the previous register. So what we need is
get that register by cursp() after pop(), then recover the reservation
by push().

fix #1542

Set regs to stack on return of const_get calls before use

Add some methods to Symbol

remove unused value for mrb_gv_remove

fix no multiline strings in mirb

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