Closes #274. Remove src/init.c and use the newer version of compileAt…

[DavisVaughan]

…tributes()

[eddelbuettel]

@johnlaing @armstrtw Anybody seconding this?

[johnlaing]

RcppExports is all magic to me, I don't really understand any of this. If it's good to you I certainly don't object.

[eddelbuettel]

Hehe. src/RcppExport.cpp and its sibbling in R/ are surely magical, and very convenient.

Here the issue simply is/was that we had hand-written / generated by a tool from R code for registration in src/init.c, and by now compileAttributes() does this for us and adds it to src/RcppExports.cpp. The PR simply updates to this.

So ... merging.