Allow the `jekyll server` command to work

Changes the default configuration to allow the `jekyll server` command
to work as expected. Preserves the behaviour of the existing scripts
`build-preview` and `build-for-publication`.
Update .gitignore
Remove debugging text from site_structure.rb

Debugging messages inadvertantly left in this plugin causing unwanted
console output during build.
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Merge and close #12 by passive consensus
Merge and close #13