Fix build

Wanted to add this PR now to kick in Travis and we will be fixing along the way.

Including helper methods to verify if its mono runtime and if its running on windows as it can be used later on. 

The tests were using string constants for file path so because you want to run in both windows and linux we must use Path.combine.

I could not test outcome of the large file tests as it is taking tremendous time to run this on my machine, lets see whats travis is saying.

Now First run I guess travis will fail because of what you mentioned in #12 missing libsodium-64.dll lets correct here.