Remove RCurl, data.table and maptools dependencies

[richardellison]

This PR removes the RCurl and data.table dependencies by replacing RCurl with httr, data.table calls with dplyr::bind_rows and maptools::spRbind with raster::bind.

[richardellison]

This latest commit is failing because of the reference to maptools in line_midpoint(). @Robinlovelace Should I reinstate the maptools dependency or is there an alternative?

[Robinlovelace]

Ah good point. No quick replacement so please re-instate. Looking at the imports for maptools it seems that there's nothing not already in there other than foreign perhaps: https://cran.r-project.org/web/packages/maptools/index.html

RCurl also removes the bitops dep. In longer run we can look at removing that maptools dep but not a priority before next release.

[richardellison]

maptools also imports lattice. The latest commits adds maptools back in.