[MXNET-531][Work In Progress] MNIST Examples for Scala new API

[lanking520]

Description

This PR contains examples written from the new API.
@nswamy @yzhliu @andrewfayres
All examples pushed here are tested locally on my Mac. It lives in the new package called org.apache.mxnet.examples

Checklist

Essentials

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[nswamy]

@lanking520 please break into one example per PR with testing and documentation included

[lanking520]

@nswamy Done that, this one MNIST specifically

[marcoabreu]

This will break on windows (I know we don't support it yet) and requires wget. Are there any scala-tools that allow downloading files with features like retry-mechanisms etc?

[marcoabreu]

We should try to avoid using the Process-Method and rather rely on language-specific tools

[lanking520]

Good idea, Will try change into this: https://stackoverflow.com/questions/24162478/how-to-download-and-save-a-file-from-the-internet-using-scala

[lanking520]

Abandon this PR since it includes too much conflict with Master Branch, open one here: #11250