Update install script links

This PR updates a bunch of links in the Agent install scripts.

- addresses issue #584
- link to Basic Agent Usage Guide, which may provide the user with enough information to solve the problem themselves
- use support@datadoghq.com instead of help@datadoghq.com

This fix doesn't require an Agent release; the new script will be presented to the user as soon as we merge to master.

@MisterRayCo @remh @LotharSee look good?