Compress data 128 and uses system.file 122

[DimmestP]

Closes issue #122 and closes issue #128
- gzipped inst/extdata/*.txt
- add cowplot
- Cq replaces Ct in filenames
- read .txt.gz in vignettes
- Changes file names in example read_lightcycler function docs
- uses system.file in all extdata imports (issue #122)

I am not sure if I understand the second point in #122:
"Vignette needs ggplot2 loaded to run. For clarity, I might call library(cowplot) in the first code chunk too."

library(cowplot) is called in the first chunk already but this chunk is hidden from the vignette, should we just make it visible?

[ewallace]

Happily approving this. It is wonderful to have the automatic checks on github and know that the vignettes re-build ok.

I moved the point about library(cowplot) to its own separate issue, #137.