Mention api debugging improvements in change summary for 0.13

Mention both #677 and #793 which introduced all functionality needed
for api debugging.
Convert table in cmd line reference to a real rST table.

Previous table was mix of reStructuredText and html produced by
automatic conversion tool. The result was rather unreadable and very
hard to edit.

I converted the table to native reStructuredText's table.
Add `sbt.extraClasspath` to command line reference.
Document changes to dependency trackin in incremental compiler.

Mention distinction between dependencies through inheritance and member
reference. Add an example which demonstrates the difference a little
bit more concretely by using some simple Scala source code.
Add documentation for api debugging feature.

Include a new section in `Understanding-incremental-compilation`
document which explains in detail how to turn on and use the
new api debugging feature.
Merge remote-tracking branch 'inc-documentation' into 0.13