Add Japanese translation of word-based periods

Hi, 

I added Japanese translation for word-based periods.

During coding, I found a problem which parsePeriod throws an exception when all separators are empty strings (this is the case for languages which lack spaces between words like Japanese and Chinese). I fixed this bug, and added some unit tests to ensure parsing Japanese like "2日5時間" is correctly handled while in existing languages, ill-forms like "2 days and 5 hours " (space is appended) are still treated as invalid inputs.

I'm very glad if you accept this pull request.

Best,
Hajime

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Add Japanese translation of word-based periods

This commit adds Japanese translation of word-based periods and
support for parsing the strings of languages lacking word spacing.
The following files are added or modified.

- messages_ja.properties

- PeriodFormat#wordBased:
  Add available languages in the Javadoc description.

- PeriodFormatter#Separator#parseInto:
  The previous impl. fails when any separator is an empty string.
  This commit fixes the problem.

- TestPeriodFormat:
  Add unit tests for Japanese.
  Also add tests whether parsing correctly throws an exception
    when a string has rendundant separators
    AND seperators for the language are not empty.

- pom.xml: Add Kasper Laudrup (author of Danish translation)
  and Hajime Senuma (author of this commit).