Write tutorial vignettes

[andrechalom]

We need to write clear package vignettes to document the use of the package. Specially the importation of spatial data is too complex to be handled by a package function, a how-to would be more adequate.

[andrechalom]

The use of shapefiles and kmz should be included in the vignette, not in the package (closing #7).

[adalardo]

during "installing the packages to build vignettes" there is an error with rmarkdown. I include rmarkdown in suggests field of description file as indicated by yihui/knitr#1864, it works.