Improvements to the build system

[jgsogo]

Hi! I've been working on the CMake files and also I'm adding some CI files to test I was not breaking anything. I'm opening this PR to start talking about it, but it is not ready to merge yet.

Things here:

• CMake to use targets (simplify properties/directories associated to libraries)
• Let the user choose Boost shared or static
• Add CI files for Travis (very simple one, but almost work: https://travis-ci.org/github/jgsogo/FreeLing/builds/681663782)
• Add CI files for Appveyor (not running yet, I cannot test Windows at home now before running CI)
• CI uses packages from Conan Center