cmake support

[gubatron]

Adds a cmake build script that:

• builds underscore shared library
• builds underscore static library
• sets a starting point for more formal tests (ideally coded with Catch2)
• builds tests using shared library
• builds tests using static library

To use type: cmake . make

Use make clean to clean the build.

[gotham13]

Thanks a lot, @gubatron . I was thinking of doing it myself but was not able to write CMakeLists.txt properly. Tested and merged