Reuse auth tokens when possible

Performance gain.

### Before
```
2.1.1 :108 > Benchmark.realtime { 10.times { GoogleCells::Spreadsheet.get("0Ap_IQ_MsilJCdDJlLTNFSEE0d3A0UElXV0stc29TQmc") } }
 => 8.336039
2.1.1 :109 > Benchmark.realtime { 10.times { GoogleCells::Spreadsheet.get("0Ap_IQ_MsilJCdDJlLTNFSEE0d3A0UElXV0stc29TQmc") } }
 => 7.08108
2.1.1 :110 > Benchmark.realtime { 10.times { GoogleCells::Spreadsheet.get("0Ap_IQ_MsilJCdDJlLTNFSEE0d3A0UElXV0stc29TQmc") } }
 => 6.573957
2.1.1 :111 > Benchmark.realtime { 10.times { GoogleCells::Spreadsheet.get("0Ap_IQ_MsilJCdDJlLTNFSEE0d3A0UElXV0stc29TQmc") } }
 => 6.051839
2.1.1 :112 > Benchmark.realtime { 10.times { GoogleCells::Spreadsheet.get("0Ap_IQ_MsilJCdDJlLTNFSEE0d3A0UElXV0stc29TQmc") } }
 => 6.527635
```

### After
```
2.1.1 :006 > Benchmark.realtime { 10.times { GoogleCells::Spreadsheet.get("0Ap_IQ_MsilJCdDJlLTNFSEE0d3A0UElXV0stc29TQmc") } }
 => 4.002985
2.1.1 :007 > Benchmark.realtime { 10.times { GoogleCells::Spreadsheet.get("0Ap_IQ_MsilJCdDJlLTNFSEE0d3A0UElXV0stc29TQmc") } }
 => 4.895547
2.1.1 :008 > Benchmark.realtime { 10.times { GoogleCells::Spreadsheet.get("0Ap_IQ_MsilJCdDJlLTNFSEE0d3A0UElXV0stc29TQmc") } }
 => 4.216413
```