Merge coverage report each module into a single coverage report

[rghetia]

After #65 test is run for each module in its own sub-directory. The raw coverage report for each module is is separate. These reports can be merged with following bash script.

#!/bin/bash

files=`find . -name 'coverage.txt' | egrep -v '^\./coverage.txt' | tr '\n' ' '`
echo "mode: set" > $1 && cat $files | grep -v mode: | sort -r | awk '{if($1 != last) {print $0;last=$1}}' >> $1

The merged report can then be converted to html format using.

go tool cover -html=coverage.txt -o coverage.html

The issue with merging is that it doesn't work for the module which is new. Following error occurs for the new module

cover: no matching versions for query "latest"

Not sure what the root cause is but it is probably related go tool attempting to retrieve latest version of the new module from github which doesn't exists (because it is new - chicken-and-egg problem).