e2e/java: cleanstart: use dbTeardown instead of wiping out ML data

Now that server isn't sending HTML errors and gradle isn't choking on
them, we can use dbTeardown to wipe out only the Samplestack
configuration on the MarkLogic server instead of deleting the data
directory.

This makes for less upheaval in the user's environment.

Also clean up and add comments to the script.
chore(readme): update instructions for nightly, add note on cleanstart

Instruct developers to download/use the latest MarkLogic 8.0 nightly.

Make mention of cleanstart.sh script for detailed steps which are being
tested daily and expected to provide a working middle tier, with caveat
that the script will only be functional on OSX.
chore(gitignore): ignore browser/dev-tasks/e2e/protractor.conf

The protractor.conf file is created during e2e tests and should not be
in version control, so add it to a .gitignore file.
login/logout moved to /v1/session
issue #231 new get session endpoint for logged in user
fixes issue:220, non-existent question id should be 404, not 500
e2e: cleanstart: do not run if not OSX and/or no mlvm install

Echo error and exit if either not OSX or mlvm command not present in
shell.
browser(authentication): use new endpoints

Endpoints now consistently use 'session' for URL.

Remove overrides to endpoint addresses, make corresponding changes to
tests, and fix schema/validation and response-handling bugs.
accept empty comment and answer objects for ask issue 241
browser: updates to package.json