[tidy] Add some comments

[version] Bump to 1.6.20

[version] 1.6.19 handle functions with (void) correctly in refactorings

[version] Update version numbers

[version] Update to 1.6.17

[version] Update version to 1.6.16 (no HTML xref)

[naming] Rename JavaMapOnPaths() to MapOnPaths()

But still a macro in head.h...

[ci][coverage] Make explict coverage for unittests in Travis

Update to 1.6.14 (now without bootstrap and enum generation)

Bump version in c-xref.el too