extend dynamodb2.table.query_count for more args
add simple test for query_count
check scan_index_forward and limit are passed correctly for query_count
remove duplicate assert
Merge branch 'jamesonjlee-dynamodb2-query_count-attrs' into develop

Adds scan_index_forward and limit to DynamoDB table query count. Fixes #2184.

Conflicts:
	boto/dynamodb2/table.py