Add codecov.io processing with travis-ci integration. (#269)

This fix adds the codecov.io process with travis-ci integration,
so that code coverage could be processed and displayed with codecov.io.

Since go cannot use test profile flag with multiple packages,
we have to iterate through packages with `coverage.sh`.

Some consolidation could be done by combining them into one `make test`.

Will do some consolidation in a follow up PR.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

coverage.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+set -e -x
+echo "" > coverage.txt
+
+for d in $(go list ./... | grep -v vendor); do
+    go test -race -coverprofile=profile.out -covermode=atomic $d
+    if [ -f profile.out ]; then
+        cat profile.out >> coverage.txt
+        rm profile.out
+    fi
+done