Fix tests and package installation

Corrected the coverage badge in "README.md" to point correctly to
coveralls.io image endpoint.

Added "custom-logger" as new dependency in "package.json" so that
it be used in outputting to screen in a pretty way from "index.js".

Fixed an Error where package installation was invoked twice. This
was due to a "npm.installPackage" nested in another "npm.installPackage"
in ".installPackage()" in file "index.js"

Added "mkdirp" as new dependency in "package.json" to allow creating
of non-existent directories in the destination path once
`.copyDirectory()` was invoked from "lib/cache.js". This allowed
a test in "test/test.cache.js" to pass.

`.storeIntoCache()` in "lib/cache.js" did NOT evaluate the targetted
package in the node_modules correctly. This is fixed allowing a test
in "test/test.cache.js" to pass.

Unnecessary module imports in "lib/index.js" have been removed.

Installing using `.installPackage()` from "lib/npm.js" to install package
from online npm registry spawns a new child process. This is implemented
in "lib/npm.js"

Few tests in "test/test.integration.js" have been clarified better.