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).
Merge remote-tracking branch 'hajimes/period_ja'
Accept pull request

Fixes #9