Make modelr installable as a devtool

[soumyaray]

Find out what else we have to do to make devtools installable via:

install_github("ISS-Analytics/modelr")

Some reading on R packages:

• http://r-pkgs.had.co.nz/description.html

[soumyaray]

btw, I notice now that our structure function collides with base::structure and masks it in the global environment. still trying to resolve what to do about this. i've posted this question on reddit's rstats group. looking for any feedback on the issue.