More elegant knitr support and bug fix

More elegant knitr support and bug fix

Implement a more simple and elegant integration of the R package knitr. Now,
lyxknitr.R does not need to move or copy files at all.

This also fixes a bug: when /tmp was on a different file system (e.g.
encrypted home), lyxknitr.R failed to move files to /tmp because it relied on
R's 'file.rename' function, which in turn relied on the rename function in
<stdio.h>, which was failing with the EXDEV errno.

Patch from Yihui Xie.