Adding mini_portile following nokogiri implementation.

This adds mini_portile to the project following nokogiri implementation.

Mini_portile embeds needed libraries for the compilation process to first control the versions that works with this gem and second as on option to allow this gem to work in environments like heroku, mentioned in #2 .

Of course there is an option to compile using system libraries: 
   gem install nokogiri-xmlsec -- --use-system-libraries
or
  bundle config build.nokogiri-xmlsec --use-system-libraries