Added responsibilities to the account manager job description.

Sales team, please take a look at this write-up, and provide your comments and suggested edits. Once you feel that this is a good representation of the various responsibilities that you tackle, we'll let it go live and place links to this description from the team page (see Job's profile on the team page for an example).

\cc @sytses @Haydn @jwrede @richardp @philcamillo @EmilyKyle @ronstroschein @ChadMalchow