Added galician (gl) locale

[oscarotero]

related #19

[hustcc]

Please add testcase, thank you.

You can just see the issue for HOWTO. #19

[oscarotero]

Done.
In local, I have a lot of test failures, in all locales. Example:

not ok 1312 should be equal
  ---
    operator: equal
    expected: '5月前'
    actual:   '4月前'
    at: module.exports (/Users/oscarotero/www/timeago.js/tests/locales/zh_CN.js:49:5)
    stack: |-
      Error: should be equal
          at Test.assert [as _assert] (/Users/oscarotero/www/timeago.js/node_modules/tape/lib/test.js:212:54)
          at Test.bound [as _assert] (/Users/oscarotero/www/timeago.js/node_modules/tape/lib/test.js:64:32)
          at Test.equal.Test.equals.Test.isEqual.Test.is.Test.strictEqual.Test.strictEquals (/Users/oscarotero/www/timeago.js/node_modules/tape/lib/test.js:347:10)
          at Test.bound [as equal] (/Users/oscarotero/www/timeago.js/node_modules/tape/lib/test.js:64:32)
          at module.exports (/Users/oscarotero/www/timeago.js/tests/locales/zh_CN.js:49:5)
          at files.forEach.file (/Users/oscarotero/www/timeago.js/tests/test.js:18:37)
          at Array.forEach (native)
          at fs.readdir (/Users/oscarotero/www/timeago.js/tests/test.js:13:11)
          at FSReqWrap.oncomplete (fs.js:135:15)

But travis execute the tests successfully, not sure why. Note that the error is in the number not in the text. Maybe is related with the timezone issue #136 ?