Emitter: Output space after an alias mapping key (#185)

Before:
    a: &b x
    *b: c

Now:
    a: &b x
    *b : c

Passing tests:
* 26DV: Whitespace around colon in mappings
* E76Z: Aliases in Implicit Block Mapping
* X38W: Aliases in Flow Objects

Test manually because `make test-suite` will overwrite changes in the skiplist:

    ./tests/run-parser-test-suite tests/run-test-suite/data/26DV/in.yaml | ./tests/run-emitter-test-suite
    ./tests/run-parser-test-suite tests/run-test-suite/data/E76Z/in.yaml | ./tests/run-emitter-test-suite
    ./tests/run-parser-test-suite tests/run-test-suite/data/X38W/in.yaml | ./tests/run-emitter-test-suite

Or edit tests/run-test-suite/test/blacklist/libyaml-emitter and do:

    (cd tests/run-test-suite; prove -lv test)

Also I added some newlines to yaml.h to help identifying states by number.