added provider of author name and email address obtained from first commit

[masakimu]

Hi, I'm a user of your license maven plugin and git plugin.

I wanted to insert original author name and email address into license headers using git history,
so I made following patch for git plugin:

• added CopyrightAuthorProvider to treat author information of first commit
• modified GitLookup.java to obtain author information from git history
• separated getGitLookup method from CopyrightRangeProvider to GitPropertiesProvider base class.
• added unit test of CopyrightAuthorProvider

Could you review this PR when you're available?

Thank you.