Review default LC_ALL setting

[hadley]

Now that all cran check machines run in UTF-8.

[yihui]

I did not know devtools was forcing the locale to be C, which is a bad idea. I believe it makes a lot of sense to remove this restriction. The locale can be temporarily set to C in a few cases to ensure consistency, such as sorting, but in general, the C locale will make R unable to read/write Unicode characters correctly.

[hadley]

I made it C a long time ago because that's what CRAN used to use - and I had a horrible problem that took hours to resolve. But I now think that time has passed :)