brokenstick 2.1.0

Improvements suggested by Journal Stat Soft.

• Defines an model.frame.brokenstick() function that adheres to conventions
• Changes return values by fitted() and residuals() to vectors
• Defines a less verbose print.brokenstick() helper
• Make all calls to library() to character argument
• Removes library(lme4) from code to evade changing the search path
• Updates vignettes/bibliography.bib to title case

Other changes

• Reorganises the vignettes
• Renames brokenstick-article.Rmd to manual/manual.Rmd, include high-res version on the site and take out of the package to save space
• Removes superfluous navigation from vignettes
• Shrinks the size of brokenstick object by removing the formula list element
• Shrinks the size of light objects by removing the sigma2j vector from the light brokenstick class