Add self-contained gometalinter build tooling.

tools/vendor/github.com/alecthomas/gometalinter/CONTRIBUTING.md

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+### Please only report errors with gometalinter itself
+
+gometalinter relies on underlying linters to detect issues in source code.
+If your issue seems to be related to an underlying linter, please report an
+issue against that linter rather than gometalinter. For a full list of linters
+and their repositories please see the [README](README.md).
+
+### Do you want to upgrade a vendored linter?
+
+Please send a PR. We use [GVT](https://github.com/FiloSottile/gvt). It should be as simple as:
+
+```
+go get github.com/FiloSottile/gvt
+cd _linters
+gvt update <linter>
+git add <paths>
+```
+
+### Before you report an issue
+
+Sometimes gometalinter will not report issues that you think it should. There
+are three things to try in that case:
+
+#### 1. Update to the latest build of gometalinter and all linters
+
+    go get -u github.com/alecthomas/gometalinter
+    gometalinter --install
+
+If you're lucky, this will fix the problem.
+
+#### 2. Analyse the debug output
+
+If that doesn't help, the problem may be elsewhere (in no particular order):
+
+1. Upstream linter has changed its output or semantics.
+2. gometalinter is not invoking the tool correctly.
+3. gometalinter regular expression matches are not correct for a linter.
+4. Linter is exceeding the deadline.
+
+To find out what's going on run in debug mode:
+
+    gometalinter --debug
+
+This will show all output from the linters and should indicate why it is
+failing.
+
+#### 3. Run linters manually
+
+The output of `gometalinter --debug` should show the exact commands gometalinter
+is running. Run these commands from the command line to determine if the linter
+or gometaliner is at fault.
+
+#### 4. Report an issue.
+
+Failing all else, if the problem looks like a bug please file an issue and
+include the output of `gometalinter --debug`