add calibration parser

[fidadoma]

the read.asc script now parses calibration results. The calibration data are parsed ar stored in data frame. Apropriate tests has been added and vignette updated

[a-hurst]

Hi @fidadoma, just letting you know that I've taken over maintenance of the eyelinker package and that I'll hopefully be merging this with my fork soon! I've rewritten the ASC parsing code and vignettes pretty extensively, so I'll need to adapt your work a bit, but it looks to be fairly straightforward.