pom: Increase version to 0.7-SNAPSHOT.

Added tag release-0.6 for changeset 4ee448adbc5a

pom: Release version 0.6.

CHANGES: Update.

ListFileTest: Add some unit tests for list file generation.

Assembler: Show version information when run without arguments.

SourceFile: Change the source file encoding from ISO-8859-1 to UTF-8.

This is a back compat change, however the impact should be low; I don’t expect
anyone to be using non-ASCII characters in strings, since no Z80 platform uses
the ISO-8859-1 character encoding anyway. And for comments the file encoding
does not really matter either way.

SourceBuilder: Simplify END*TERMINATORS construction a bit by using varargs.

pom: Add <release> tag to ensure we’re only using Java 8 APIs.

pom: Update Maven plugins to latest versions.

README: Document END directive.

Has been supported since the first release, but was never documented.

SourceTest: Improve test for END directive.

SourceBuilder: Make global scope the parent of the top-level source scope.

Otherwise, labels with a name matching a globally defined symbol could not be
defined on the top source level.

Thanks to Santi Ontañón Villar for reporting.
See https://www.msx.org/forum/msx-talk/development/glass-z80-assembler?page=15#comment-385460

SourceBuilder: Introduce another constructor with standard end terminators.

SourceBuilder: Express constructor in terms of other constructor.

SourceBuilder: Private END*_TERMINATORS and constructor w/ terminators argument.

Source: Remove constructor that assumes global scope.

SourceTest: Add test for END.

Parser: Support '' and "" escapes in character and string literals.

README: Escape sequences also work in character literals.