Adding --force param to img compressor. Small refactorings.

Solving [Issue 35](https://github.com/ligershark/AzureJobs/issues/35).

Included in this change:

- included code to handing a new cmd line param `--force`. This will force all files through the compressor regardless whether they're in the local img compressor cache.
- variable name refactoring for clarity (e.g. there are 2 log files involved in the img compressor)
- small methods. Xml doc comments clean up. 
- code style with curly brackets; put on same line K&R style as discussed.