[clean-up] FileSlicer.slice! renamed to FileSlicer.remove!

* Consistent with FileSplicer
* Consistent with FileSlicer.remove method
[clean-up] typo
[clean-up] No longer need redactor

Was thinking of processing file and removing <section>s in a
'government redaction' style, but decided to keep it as simple as
possible with the FileSlicer approach.
[clean-up, rake] rake :clobber now handles reveal.js git submodule
[clean-up] slides.haml now accurately generates first two slides of reveal.js
Merge branch 'clean-up'