Update CheckoutTask.java

JENKINS-24741 : Expose duration of events.
Adding simple duration code generates:
SCM Task: restoring all missing and changed revisions.
... [list] = reconcile [-n, -e, -d, -f]
SCM Task: Prepare duration : 52 s
SCM Task: syncing files at change: 74227
... sync C:\Program Files (x86)\Jenkins\jobs\b2\workspace/...@74227
... force update false
... bypass have false
SCM Task: Sync duration : 0 s

JENKINS-25648 : P4_CHANGELIST variable
Adding a local properties file solves the problem better.
I can set my build name for example to
#${BUILD_NUMBER}.${PROPFILE,file="p4-plugin.properties",property="buildChange"}

or use the env-inject plugin to use it later.

We could add other items i suppose
Merge pull request #8 from mjoubert/patch-2

Update CheckoutTask.java
I'll do some minor formatting changes later, but the changes look good.