Extends the web3-eth module with the method getPendingTransactions

[nivida]

Description

This PR does add the JSON-RPC method eth_pendingTransactions as getPendingTransactions method to the web3-eth module.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success and extended the tests if necessary.
• I ran npm run build-all and tested the resulting file/'s from dist folder in a browser.
• I have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the live network.

[coveralls]

Coverage remained the same at 84.467% when pulling a0efc21 on feature/getPendingTransactions into 335e8e6 on 1.x.

[cgewecke]

Awesome. The types are already defined here...everything looks good to me.