uses class extends, code restructure need major release

[uniibu]

This is in relation to #9

Major/Breaking changes:

• No longer mutates the original pg classes pg.Pool and pg.Client
• Uses class extends whenever possible.
• To use pg-extra's helper methods, user's must initialize it using new pg.extra.Client or new pg.extra.Pool
• In relation to above, pg.Client and pg.Pool now returns to their respective original instances.
• removal of parse-url since pg can accept { connectionString }

Minor changes:

• Restructuring of codes.
• Renaming of files using camelCase
• Update of test files to use pg.extra.Client and pg.extra.Pool
• Adds nodejs v8 and v10 to travis test.

Passes travis tests.

Note: This does not update package.json version and readme.md. So if merged, you will have to update them accordingly. :)

[danneu]

Awesome! I won't be able to review this for another few weeks. Currently traveling.

Thanks for the work!

[danneu]

Merged. Now I need to sync the readme and prep a major release.