[v3] c++0x common_type, <ratio> and <chrono>

["Chris Fairles" <chris dot fairles at gmail dot com>]

Attached is a patch that implements most of n2615. Whats the word on
its acceptance status? I haven't made full blown test cases and such
for anything in <chrono>. The clocks aren't portable. I just put them
in as placeholders that "worked for me" since I'm not really sure how
to go about integrating posix/linux specific functions.

Pending some feedback, I can tidy up the patch, add testcases etc.but
I want to make sure that some variation of n2615 has in fact been
accepted into the std.

Chris

Attachment: patch_ratio_chrono.txt
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