#820 added proto support

[romanchechyotkin]

Description

Sending logs to Loki using protobuf messages
Fixes #820

Also set up proto as default format because

BenchmarkLokiJSONRequestBuilding/SmallBatch_10_events-10         	   23757	     50821 ns/op	  144009 B/op	    1650 allocs/op
BenchmarkLokiJSONRequestBuilding/MediumBatch_100_events-10       	    2210	    530140 ns/op	 1463461 B/op	   16417 allocs/op
BenchmarkLokiJSONRequestBuilding/LargeBatch_1000_events-10       	     204	   6455180 ns/op	15090684 B/op	  164050 allocs/op
BenchmarkLokiJSONRequestBuilding/VeryLargeBatch_10000_events-10  	      15	  81413203 ns/op	149441513 B/op	 1641516 allocs/op
BenchmarkLokiProtoRequestBuilding/SmallBatch_10_events-10        	   25628	     48096 ns/op	  141786 B/op	    1546 allocs/op
BenchmarkLokiProtoRequestBuilding/MediumBatch_100_events-10      	    1904	    592796 ns/op	 1413570 B/op	   15407 allocs/op
BenchmarkLokiProtoRequestBuilding/LargeBatch_1000_events-10      	     170	   7107666 ns/op	14111673 B/op	  154026 allocs/op
BenchmarkLokiProtoRequestBuilding/VeryLargeBatch_10000_events-10 	      16	  76681576 ns/op	140987275 B/op	 1540086 allocs/op
BenchmarkLokiJSONvsProto/JSON-10                                 	     208	   5690060 ns/op	15089443 B/op	  164039 allocs/op
BenchmarkLokiJSONvsProto/Proto-10                                	     249	   4754119 ns/op	14121091 B/op	  154021 allocs/op
PASS