Integrity and metadata fixes #152

[ewallace]

fixes #152 https://github.com/ewallace/tidyqpcr/issues/152#issue-1205925988

• DESCRIPTION, Depends on tidyr and fix version 0.4. (kept Imports on tibble because it makes the examples nicer and we use tibble in a LOT of examples)
• added files to .Rbuildignore
• replace LICENSE with LICENSE.md
• load packages in tests with dplyr:: in front of relevant functions.
• tidied test code, mostly linting / indenting
• moved "getting started" further up in README.md
• added top-level help file