Add SPDX license identifier and update copyright year

[dreamer]

SPDX (Software Package Data Exchange) identifier is machine-readable field used propagating license information - it's very useful for cases where the project consists of many files using slightly different licenses.

It also makes it easier for users to notice what license is being used, in specific body of work.

Read more: https://spdx.org/licenses/

Also, separated copyright holder name from the copyright year using two spaces (as examples in GPL text suggest to do) - this makes it easier to update copyright years via regular expressions.