Fix: flush() now calls libftdi(>= 1.5) tc[io]flush functions.

[syha6821]

flush() now calls libftdi >= 1.5 tc[io]flush functions.
Closes #13

[codedstructure]

Thanks for this PR - looks useful and I appreciate that you've kept the code similar to the existing code.

One request though - as implemented the tests (e.g. make test) will typically fail. I also wonder if it should attempt the newer functions first (or even only?) since libftdi1.5 has been out a long time now.

[syha6821]

I’ve amended the previous commit.
I’ve included the test code as well, and since the flush-related implementation in versions earlier than 1.5 contains bugs, it makes sense to support only version 1.5 and above, so I omitted the 1.4 code.