Adding logging (issue #92)

[toolforger]

First commit is a toe in the water: cache bypass decisions are logged.

PR created so we can discuss whether that's the right direction to take.
I have no idea about testing this. I'm sceptical about testing logging, because it tends to be both volatile and inconsequential so the gains of testing are marginal, but I'm happy to follow whatever the policy is.

[landscape-bot]

Repository health decreased by 0.56% when pulling 54e3304 on toolforger:add-logging into bc89ecc on ionrock:master.

• 1 new problem was found (including 1 error and 0 code smells).
• 1 problem was fixed (including 0 errors and 0 code smells).

[toolforger]

Right, that global statement will go. (Not an error actually, but it's gratuitious.)

[landscape-bot]

Repository health increased by 0.15% when pulling 075545f on toolforger:add-logging into bc89ecc on ionrock:master.

• No new problems were introduced.
• 1 problem was fixed (including 0 errors and 0 code smells).

[ionrock]

@toolforger Thanks for the PR. I merged this in the branch for #100. It should be merged into master when the doesitcache tool is finished.