Dynamically load libnetfilter.so so that applications who use the library will load on all systems.

[arunmk]

We should be able to use the library even on systems where libnetfilter is not available. There must be a way to check if we can use libnetfilter, and if we cannot, we should be able to gracefully handle the situation.
With the current implementation, a binary using this library will fail while loading.

[arunmk]

This will be fixed by pull: #6

[arunmk]

@kraman please advise if I should do anything else. I have run a few tests on my machine with this change.

[kraman]

@arunmk thanks for the PR, do you mind sending your tests as well, I have been meaning to add tests to this repo.

[arunmk]

@kraman sorry I was unclear about the tests. These have been manual tests with a debugger. I can create a few tests as a new PR though.