vendor: add all of our dependencies

vendor/github.com/gregjones/httpcache/README.md

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+httpcache
+=========
+
+A Transport for Go's http.Client that will cache responses according to the HTTP RFC
+
+Package httpcache provides a http.RoundTripper implementation that works as a mostly RFC-compliant cache for http responses.
+
+It is only suitable for use as a 'private' cache (i.e. for a web-browser or an API-client and not for a shared proxy).
+
+**Documentation:** http://godoc.org/github.com/gregjones/httpcache
+
+**License:** MIT (see LICENSE.txt)
+
+Cache backends
+--------------
+
+- The built-in 'memory' cache stores responses in an in-memory map.
+- https://github.com/gregjones/httpcache/diskcache provides a filesystem-backed cache using the [diskv](https://github.com/peterbourgon/diskv) library.
+- https://github.com/gregjones/httpcache/memcache provides memcache implementations, for both App Engine and 'normal' memcache servers
+- https://github.com/sourcegraph/s3cache uses Amazon S3 for storage.
+- https://github.com/gregjones/httpcache/leveldbcache provides a filesystem-backed cache using [leveldb](https://github.com/syndtr/goleveldb/leveldb)