CRAN release version 0.3

Accepted for rOpenSci:
ropensci/software-review#115

The following suggestions from the reviewers were added:

• email validation, thanks @sckott
• bugfix Shiny Addin, thanks @tts
• add version number as package dependencies for shiny, thanks @rossmounce
• add unit test using lintr package, thanks @maelle
• improved documentation