Some small improvements

We've so far had our own branch of the plugin in use and would now like to close it. There were some bigger and smaller changes from which I've picked the ones that we'd like to keep. Would it be possible to get these integrated to the official plugin?

- Kill p4.exe if the build gets aborted. 
- Contribute PERFORCE_DEPOT, PERFORCE_STREAM, PERFORCE_CHANGELIST_SOURCE and PERFORCE_CHANGELIST_SOURCE_DATE to the environment. PERFORCE_CHANGELIST_SOURCE is exactly the same as P4_CHANGELIST.
- Ignore errors/warnings 'no such file(s)' during sync. This change could cause problems to some as it changes the behaviour. 
- Check stream existence instead of a path.
- Minor help text improvements.