Support vagrant cachier and Ubuntu trusty box.

Changes the box used for precise for the one already avaiable and used
by default on older Vagrant installations.
Add composer task.

Enables installation of composer and global composer packages, this
option is enabled by default.

Details on the documentation were added relating composer and also the
main variables to be configured by playbooks outside the role. A better
example of playbook using the role is also made.

Tasks relating composer and fpm were put in another role file, making
them more easible managed.
Merge pull request #13 from augustohp/composer

Composer and documentation.