Use SHA256 constant when available

Should help with HHVM, re #153
Check curl_exec as well as curl_version

I think I've seen cases where web hosts have got Curl installed, but disable it via disabling the function curl_exec (the function that actually causes network activity). In this case, the check for curl_version alone will give a misleading result. Hence this tweak.
Merge branch 'master' of https://github.com/google/google-api-php-client
Merge pull request #156 from ianbarber/master

Use SHA256 constant when available
Merge branch 'master' of https://github.com/google/google-api-php-client
Remove large test file

This probably shouldn't have been checked in in the first place!
Merge pull request #159 from ianbarber/master

Delete big test file
Merge pull request #157 from DavidAnderson684/patch-2

Check curl_exec as well as curl_version