add directive 'mruby_set' and 'mruby_set_inline'

These directives assign a result of mruby script to a variable.

mruby_set_inline $res1 "ARGV[0] + ', ' + ARGV[1]" "Hello" "World";
mruby_set        $res2 "/etc/nginx/script/hello.mrb";
Merge pull request #45 from cubicdaiya/feature/mruby_set

add directive 'mruby_set' and 'mruby_set_inline'
Support current mruby build system.
Update README and removed mrblib.o from Makefile.in
output a error log when mruby script does not exist for mruby_set

a missing mruby script for mruby_set causes a failure of starting nginx.
In this case ngx_mruby should output a error log
Merge pull request #47 from cubicdaiya/issues/no_error_log_in_mruby_set

output a error log when mruby script does not exist for mruby_set
Update README.md
Merge branch 'master' of git://github.com/charlescui/ngx_mruby into charlescui-master
Cosmetic changed from #44
prototype declaration is not found

The prototype declaration of ngx_mrb_set_request_uri is not found.
remove unneeded comment
Merge pull request #51 from cubicdaiya/issues/unneed_comment

remove unneeded comment
Merge pull request #50 from cubicdaiya/issues/no_prototype_declaration

prototype declaration is not found
add getters/setters and define macro for request members

request members is following

 * request_line
 * unparsed_uri
 * method
 * protocol
 * args
Merge pull request #53 from cubicdaiya/feautre/add_request_member

add getters/setters and define macro for request members
fix bug(Nginx.rputs empty string causes response-stalling)

example is following

Nginx.rputs("abc")
Nginx.rputs("") or Nginx.rputs(nil)
return Nginx::HTTP_OK
add getter and setter for r->headers_(in|out)
Merge pull request #54 from cubicdaiya/issues/rputs_empty

fix bug(Nginx.rputs empty string causes response-stalling)
Merge pull request #55 from cubicdaiya/feautre/setter_and_getter_headers_in_out

add getter and setter for r->headers_(in|out)
cleanup root directory

move *.(c|h) to ngx_mruby/