Improve include system

[DavidJourdan]

I've made a few modifications to make it easier to use libshell as an external library:

• The CMakeLists.txt now conditionally builds the examples and tests depending on wether libshell is the top-level project or not. Adding libshell as an external dependency should now be as simple as writing add_subdirectory(path/to/libshell) in your CMakeLists file
• The header files are now in a libshell directory, which means we have to write #include "libshell/File.h" instead of #include "File.h", helps with disambiguation when having multiple dependencies

I was also thinking about renaming the namespace LibShell to lowercase libshell since this seems to be the standard practice, but this could break some existing code so I haven't done it yet