A release for DOI

This release serves as the requirement of enabling a Digital Object Identifier (DOI) from Zenodo.

Add Manure data into the dataset

• Added manure data from 1982 to 2012 (every five years).
• Update the column names: Fertilizer -> Nutrient.
• Added another column: Input.Type to indicate the nutrient input type.
• Update readme and related articles.
• Fixed the failed build of vignettes.

First CRAN release

Cleaned up all the dataset (missing 1986), packed as an R package and published in CRAN.