Windows support

I've made some updates to accommodate Windows. Mostly checking the active OS and using the correct lib/header file names. I've also exchanged the ffi-inliner stuff with some code that should be just as portable without requiring the compilation. This, again, was to ease things on Windows. You may still need a ruby dev kit to install all the gem dependencies, but after that you're good to go.

A note about the tests, they all pass but I have to run them one file at a time on Mac/Linux to avoid double-free errors. On windows there seems to be no problem with just running `rspec` - they all run and pass just fine (aside from the occasional segfault). Is this a known problem? Or possibly, "my problem?" Oh, yeah, and I've changed some of the values in the tests to correspond to the default verax systems snmp simulator configuration (noted in the README.md file in the specs folder). If you don't want these changes, I can remove them.

I've also extended the library with a first stab at SNMP agent support, which works well at least for my purposes. If we're able to get this pull into an acceptable state and merged, I'll be happy to send another pull request with those additions. As an alternative, I could package it as another gem that simply depends on net-snmp.

Any time to review this request and provide feedback is much appreciated! Thanks for the great library!