Fix file headers

[wzorgdrager]

File headers are currently a bit inconsistent (license, no license, default intellij header), we should change them all to the same format.

[nvankaam]

Yes, they should all have the apache 2.0 license.

[wzorgdrager]

Also the test files?

[nvankaam]

According to @gousiosg "all files should include the Apache header", so I assume so yes.