add a instance variable for saving headers, avoid compute headers again.
make ngx_mruby can read nginx build in variable and request variable.
Add Kernel#server_name
add ngx_mruby read ngx build in variables and request parameters.fix example nginx.conf error for headers.
Merge pull request #56 from charlescui/master

ake ngx_mruby can read nginx build in variable and request variable.
Cosmetic changed. Replace [tab] to [space * 2] and adjust indents.
Cosmetic changed. indent space from 2 to 4.