Add Gradle wrapper in the repository

[lacasseio]

Is your feature request related to a problem? Please describe.
It's not obvious which version of Gradle the project should be using as well as makes all development configuration more complicated.

Describe the solution you'd like
The Gradle wrapper should be added to the repository. For security concern, always verify the checksum of the wrapper when accepting PR that includes one.

Describe alternatives you've considered
The alternative, which is not providing a wrapper. Adds maintenance burden on the users and possibly on the team when answering questions regarding Gradle version to use. It's also the most idiomatic way of using Gradle.

Additional context
Feel free to raise any concern about using the Wrapper. The Gradle team is happy to address them.

[lacasseio]

See PR: #456

[ryanmkurtz]

Closing for now. See comment in pull-request for more info.