Updated code to newer standards

- use Cask for package management

- use `s` and `dash` for strings and lists manipulations

- consistently use `start-file-process` to run commands, which avoids
  quoting arguments

- change the squeue output parsing strategy: now `slurm-squeue-format`
  contains a high-level description of the output format, which is used
  to parse columns. WARNING: the columns width must be defined in such a
  way that all jobs fit in the format (otherwise columns are unaligned).

  This should help with issue #1