Rm warnings, rfactor code

Impl tut5

Merge pull request #1 from snowzurfer/dev

Impl tut4, rm exceptions use

Impl 3rd kaleido tut; use variants, impl Visitor, Driver

Rfctor to use exceptions instead of returning `nullptr` in parser

Impl. use of `std::variant` for the token class value

Impl. use of `std::variant` for AST instead of inheritance

Impl. support of C++17; updt README, CmakeLists, ycm_extra_conf

Add README.md

First commit, impl 1+2 exercise, classify exercises