Move curl version check to the beginning of module init.

No reason to do any other work if this check fails.
Raise an ImportError when libcurl version requirement fails rather than nuking the interpreter.

Partially implements #113.
Include wanted and actual version in the exception message
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
Treat "none" and "other" the same; this unbreaks windows builds with winssl
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
Merge pull request #121 from p-push/runtime-ssl-check

Runtime ssl check
Support building libcurl against zlib