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