datastore: allow nested unindexed properties

[stephenplusplus]

Fixes #1916

To Dos

• Docs
• System tests
• Unit tests

This will allow excluding embedded properties from indexing, using the solution discussed here. Here is the sample from our system tests:

datastore.save({
  key: datastore.key('Kind'),
  data: {
    longString: longString,
    notMetadata: true,
    metadata: {
      longString: longString,
      otherProperty: 'value',
      longStringArray: [
        {
          longString: longString,
          nestedLongStringArray: [
            {
              longString: longString,
              nestedProperty: true
            },
            {
              longString: longString
            }
          ]
        }
      ]
    }
  },
  excludeFromIndexes: [
    'longString',
    'metadata.longString',
    'metadata.longStringArray[].longString',
    'metadata.longStringArray[].nestedLongStringArray[].longString'
  ]
})

[coveralls]

Coverage remained the same at 100.0% when pulling ada4d67 on stephenplusplus:spp--1916 into 105f28b on GoogleCloudPlatform:master.