Preserve directory structure

Instead of generating images directly on `outputDirectory`, use the same
directory structure as in `sourceFiles`.

Fixes #8
Merge pull request #15 from esamson/preserve-directory-structure

Preserve directory structure
Update README.md
Update README.md
#16 skip execution if source directory is missing
* early exit added if source file directory did not exist or invalid
#16 skip execution if source directory is missing
* whitespace formatting reverted
Merge pull request #17 from barthel/issue_16

#16 skip execution if source directory is missing
Prepare for release.
[maven-release-plugin] prepare release 1.1
[maven-release-plugin] prepare for next development iteration
Reflect latest versions.