bigquery/table: add copyFrom()

[stephenplusplus]

Fixes #2278

This adds a method on a Table object, copyFrom:

table.copyFrom([
  dataset.table('source-table-1'),
  dataset.table('source-table-2')
], function(err, job) {});

[coveralls]

Coverage decreased (-14.7%) to 85.312% when pulling f7dec92 on stephenplusplus:spp--2278 into 0362106 on GoogleCloudPlatform:master.

[stephenplusplus]

Just a thought, would it be better to merge this functionality into import?

table.import([
  sourceTable1,
  sourceTable2
], ...

[callmehiphop]

Just a thought, would it be better to merge this functionality into import?

Personally I like this! We'd definitely want to document very clearly what supplying a string would do for that method though.

[stephenplusplus]

For any users familiar with the BQ concepts of copying as opposed to importing, the separation might be appreciated. And like you pointed out, import already has a couple ways of being used, so adding another might add more confusion. For those reasons, let's see if copyFrom is good enough and always keep the door open for refactoring and reconsideration in the future.