Add scoverage + codecov report

Author: artem0

.travis.yml

@@ -5,7 +5,7 @@ scala:
   - 2.12.4
 
 script:
-  - sbt clean test
+  - sbt clean coverage test coverageReport
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)

README.md

@@ -4,6 +4,7 @@ Merkle trees are typically implemented as binary trees where each non-leaf node
 The leaves can either be the data itself or a hash/signature of the data.
 
 [![Build Status](https://travis-ci.org/arukavytsia/merkle-tree.svg)](https://travis-ci.org/arukavytsia/merkle-tree)
+[![Test Coverage](https://codecov.io/gh/arukavytsia/merkle-tree/branch/master/graphs/badge.svg)](https://codecov.io/gh/arukavytsia/merkle-tree/?branch=master)
 [![License](https://img.shields.io/badge/license-GNU-green.svg)](https://github.com/arukavytsia/merkle-tree/blob/master/LICENSE)
 
 ## API

project/plugins.sbt

@@ -1 +1,3 @@
-logLevel := Level.Warn
+logLevel := Level.Warn
+
+addSbtPlugin("org.scoverage" % "sbt-scoverage" % "1.5.1")