Move example agent into new subdir examples/
Rename example agent to simple agent

We're going to have other example agents in the examples/ subdirectory as
well, therefore the name "example agent" would be ambiguous. The MIB and its
contents have been renamed, too.
Non-functional improvements to the simple agent

simple_agent.py's heading comment block:
- Add note on running the run_simple_agent.sh script
- Add note on using sudo when starting the agent directly
- Encourage user to use own <rosecret> instead of "public"

simple_agent.py's counters code:
- Prettify sort order of code blocks
- Explain initialization value
- Document that increment() is a counter object's method more explicitly

run_simple_agent.sh:
- Mention snmpget as well
New example threading_agent.py demonstrating asynchronous data updates

The default simple_agent.py always blocked until certain events occured, eg.
SNMP requests had to be processed, and only updated its SNMP objects
inbetween. Also SNMP requests couldn't be handled while data was in the
process of being updated. Thus here's a more real life-usable example that
outsources data updating into a separate thread. It does however not address
possible locking issues.
Small update to the README

- Phrase missing API stability less dramatically
- Add TODO of SNMP notifications/traps