INTSAMPLES-119 Update Monitor for Twitter OAuth

Twitter now requires authentication.

Disable the real twitter adapter by default; enable it
with spring.profile "twitter" - requires the user to get
real oauth tokens.

Replicate oauth setup instructions from the twitter sample.

Add note to README for enabling the real twitter adapter.

Add .gitignore so committers can keep their real credentials
in, say, 'oauth.properties.foo'.

Add note about `SpringIntegrationTest`.

Update dummy tweets to use new icon.