Prevent log file line overwrite. Add percentage savings to log file.

Solving [Issue 24](https://github.com/ligershark/AzureJobs/issues/24).

Included in this PR:

- FileStream now uses FileMode.Append, FileAccess.Write
- percentage savings is included in the image . Sent along CompressionResult.Percent property 
- calculated percentage saved in the text file minifier.
- a fewer shorter methods in the Logger class. Some string consts, too.

The log files for the image and text file minification now don't overwrite on line 1, and look like this:
![output_examples](https://cloud.githubusercontent.com/assets/930731/3591738/d46ce822-0c67-11e4-9db9-d4b624cb99b9.png)