Improve logger & general string concat performance

[bbonamin]

Dramatically improves the performance of the code where strings are concatenated, by changing the "+=" for "<<".
Sample profiling of line 50 on logger.rb 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

[karmi]

Thanks, verified and merged!