pdf generation scripts for getting_started, mix_otp and meta.

As I found that the files in
https://groups.google.com/forum/#!topic/elixir-lang-core/aRzIbxswsLQ
are gone. I come up with my own scripts to generate nice pdf version
for elixir getting_started documents.

The generated results can be viewed directly as
https://github.com/liyu1981/elixir-lang.github.com/blob/master/_pdf/getting_started.pdf?raw=true
https://github.com/liyu1981/elixir-lang.github.com/blob/master/_pdf/meta.pdf?raw=true
https://github.com/liyu1981/elixir-lang.github.com/blob/master/_pdf/mix_otp.pdf?raw=true