Improve logger & general string concat performance

Dramatically improves the performance of the code where strings are concatenated, by changing the "+=" for "<<". 
Sample profiling of [line 50 on logger.rb](https://github.com/karmi/retire/blob/0512cc5e3528d9ab4c709f58a3ffb2e20c1f4196/lib/tire/logger.rb#L50) with a 20,000 lines json response from ElasticSearch:
```
Before the change
99.74% 	48.04 seconds String#each_line

After the change
74.91% 	0.11 seconds String#each_line
```

Fixes #155