First draft of codespeedization of test/perf

 * Change webhook notification URLs to julia.mit.edu (Linux) and criid.ee.washington.edu (OSX)
 * Add `codespeed` target to test/perf/Makefile, which will run perfsuite and upload result to hardcoded codespeed installation location
   * Note that `make codespeed` will attempt to `Pkg.add()` the packages `JSON` and `Curl` if not already installed.
   * Exclude `sort` subtests for now, see https://github.com/JuliaLang/julia/commit/453b4efc30bfdabf8a1db33a496ef5972afc9812
 * Annotate all @time macro calls with an extra "Description" field, used to populate the codespeed database for the first time
   * Again, exclude the `sort` tests for now, as this integration with codespeed will have to wait.  For now, empty descriptions
 * Until extra build-time information is provided, expect environment variables such as JULIA_FLAVOR, JULIA_BRANCH, and JULIA_COMMIT_DATE on the command line when `make codespeed` is invoked.