Added support to build on windows

There're minor changes to libssh src to make it build.
Also OpenSSL search path is hardcoded, see
https://github.com/TooTallNate/node-gyp/wiki/Linking-to-OpenSSL and
http://slproweb.com/products/Win32OpenSSL.html
Fix sftp-test failures for recent builds

After digging around for some time, found the fix for the test to pass.
Very likely this is due to ssh2 upgrad.
Merge pull request #37 from jamesallen426/master

Added support to build on windows