Upgrade to libuv 1.15.0

[wch]

This closes #42. I've tested it with shiny::runExample("01_hello") with Shiny 1.0.5 on macOS 10.12.6, Windows 10, and Centos 6.

Unlike the libuv-1.8.0 branch, on Windows, this builds libuv using Rtools, by simply running make -f Makefile.mingw. (Thanks to @jimhester!)

• The first commit just updates the files for libuv
• The second commit makes modifications to httpuv code so that it all works. For the second commit, many changes were borrowed from 5a4ba77.

[QuLogic]

Is the source for libuv now pristine? (I'm wondering if httpuv could be built against a system copy now).

[wch]

The copy of libuv that's included is unchanged from original sources.

[QuLogic]

Great!

[QuLogic]

Has any thought been made to building against a system copy? Would a patch be accepted?

One of the troubles with bundling is that you need to be on top of any issues in the code you're bundling. For example, the ancient copy of libuv that is in the current release (but not git) contains a CVE, 2015-0278, though I would hope that no-one runs R with elevated privileges.