Merge branch 'issue#7' into 'devel'

Issue#7: Data set and vignette

This branch:
  - Add `data-raw/` directory. In this directory, data preparation is done. The code to `@examples` section in the documentation is also done there.
  - Add R script to get `txt` files from `RData` files.
  - Add `data/ahs.RData`, a data.frame named `ahs`;
  - Modifies de data.frame and improves de documentation (CMT 0a92ba26 for details);
  - Add lattice and latticeExtra in the Suggests, as they are required in the `@examples` of `ahs`.
  - Add a vignette template that contains plots and summaries of the `ahs` data set.
  - Add build_vignette() in buildPkg.R.
  - Build the package and vignette.
  - Compilation goes without errors. Success!.


See merge request !5