Merge pull request #122 from p-push/libcurl-version-check

Move libcurl version check to the beginning of init process and raise ImportError if it fails rather than nuking the interpreter entirely
HAVE_CURL_SSL refers to any ssl library, and should be enabled when any backend is being used
Require that compile time and runtime ssl backends are the same
Test against ssl-less libcurl on travis
I will need to know what libcurl is compiled against on travis
Current travis default is openssl
Fix test suite on ssl-less libcurl
Document new ssl requirements
Treat "none" and "other" the same; this unbreaks windows builds with winssl