Add httpbin-like server in native PHP
Allow changing test host

Uses the REQUESTS_TEST_HOST env variable if it exists, or the more specific
_HTTP and _HTTPS versions. Defaults to httpbin.org.
Ignore http.log/http.pid
Use localhost for HTTP tests, not HTTPS tests
Move test server to Composer dependency
Correct path to start/stop
Use vendor/bin
Use old-style ternary for PHP 5.2
Preserve path when sudoing
Set PHPBIN to correct version
Prefer distributions with Composer

Hopefully should speed up testing slightly.
Merge pull request #107 from rmccue/speedy-tests

Speed up test running by using local server