Simplified the author, title and other PDF info by defining new commands.

These new commands have been defined:
  \newcommand*{\mytitle}{Document title}
  \newcommand*{\myauthor}{Alfredo Hernández} 
  \newcommand*{\myuni}{Universitat Autònoma de Barcelona, Departament de Física}
  \newcommand*{\mydate}{\normalsize \today}