Organize build structure and refactor code

[MoriokaReimen]

Dear Mr. Tai-Min

Your Boost-Serial-Port project got my attention and I found that this project is the best Serial communication library for my hobby arduino project. I am sure this project helps a number of hobby developers like me.

I found some room for improving this library. Therefore, I decided to make this pull request.
Through this pull request, I did some code-refactor as below.

1. Organize build structure to make it easy to embed other project structure.
2. Apply astyle -A8 code formatter
3. move template functions from *.cpp files to *.hpp file
4. Auto download gtest and build test program when ENABLE_TEST CMake option is enabled.

I hope this pull request helps you.
Thank you.

[Tai-Min]

Hello! Sorry for REALLY late response but I'm a bit busy with my master's thesis and the semester overall.
I'm grateful for your participation in this project and I'm glad that you like it! Currently I don't have Boost installed on my workstation but in the near future, I'll get more deeply acquainted with the changes you made and after this I'll merge the PR.

Thank you!