Make Agent selection autocompleter more discoverable 

Related to issue #420.
I tried to reuse most of the UI and came up with these 2 new features:
1) the autocomplete now searches both in the agent's name and description
2) when an agent is highlighted in the select box, it's markdown description is shown in a div right beside the select box.

But the new agent page gets a little slow, because I had to create instances of every Agent and the markdown had to be parsed for each of those.
What do you think about it @cantino ?