Update make, lots of linting

[jthurman42]

Update to a more verbose linting and make config.

Big-ish changes:

• Moved the cmd package into internal/cmd
• Move main.go to cmd/go-newrelic (result is binary now compiled into bin/<os>/go-newrelic

Most other changes are linting related. Lots of error checking on w.Write() (Should refactor these tests at a future date)

The api package has not been moved, so this should be backwards compatible with external users or the library, unless they are wrapping cmd which would seem like an odd thing to do.

Due to that, we might want to consider this a for a 5.0.0 release, and if so I'd argue for some more refactoring first (breaking out api into domain specific packages, moving under pkg, a bigger conversation to have)

Alternatively, we can rollback the move of cmd package and change the Makefile to deal with it.

Makefile enhancements:

• Run unit and Integration tests
• Auto-generate documentation
• Use golangci-lint for stricter linting
• Easier cross-compile, directory structure for building multiple platforms
• Travis config updated as well with ci specific make commands

[jthurman42]

This needs some serious rebase work, will reopen at another time.