Fix timezones

[penelopeysm]

There are two separate issues with timezones, one is what the app uses internally to find meeting times, the other is what it shows you when you use it.

On the former:

• meetings are only searched for during London hours 0830-1730 (previously they would search in your local timezone, which was bad)

On the latter:

• by default meeting times are shown in London timezone
• adds a --local switch to allow programme to output meeting times in local timezone
• to avoid any ambiguity, the programme now tells you what timezone it's outputting times in