Added check for files before generating SVG

If no files are included in stream, gulp-svgstore will not generate an
SVG. Addresses https://github.com/w0rm/gulp-svgstore/issues/10.
Merge branch 'development'
Added cb to return on empty stream
Added changelog to readme.md
Added test and change log entries starting 1.0.0
Merge branch '10-dont-generate-empty-svg'