s2: R bindings for Google's s2 library for geometry on the sphere

The package (master branch) currently passes R CMD check without any errors and warnings on Linux, OSX and Windows. Only a minor subset of the C++ library is wrapped at the moment. Simple R data structures such as plain matrices and lists are used. The API is not stable and changes should be expected.

The core C++ code is in src/geometry. This code is a slightly modified copy of the corresponding directory in this repo. (The subdirectories python and test are removed since they aren't needed by the R package.) The modifications to the original code are minor edits to satisfy the R package checker, and no new functionality is introduced at the C++ level.

The original source code was compiled into several shared objects with cmake, but we simply build everything into one shared object using the R standard file Makevars.in (Makevars.win on Windows) together with a configure script. This decision was based on advice given by Dirk Eddelbuettel to the RcppCAF package at stackoverflow. The configure script checks for the system requirement openssl and is based on the configure script by Jeroen Ooms in the openssl R package.