Rename the doc file extensions from .txt to .rst.

This gets us onto .rst for RBTools as the official doc extension, making
it easier to review, and allowing for more consistency between the
modules.
Switch the doc building to look up .rst instead of .txt files.
Add release notes for RBTools 0.6.

This details all the changes we made in 0.6, which is a pretty
substantial release.

Reviewed at https://reviews.reviewboard.org/r/5675/
Release RBTools 0.6.
Add missing contributors and a missing fix to the 0.6 release notes.
Post against release-0.6.x
Add TRACKING_BRANCH to .reviewboardrc on release-0.6.x
Fix documentation for 'rbt post -I'

The rbt post documentation hadn't been entirely updated for the revision
changes to the command-line parsing. This updates the main 'usage' statement,
along with the CVS/SVN section on including specific files.

Testing done:
Built HTML docs.

Fixes bug 3315.

Reviewed at https://reviews.reviewboard.org/r/5725/
Fix -I with svn repositories.

The line that added the files to the diff command line was indented four spaces
too many, which made it only work if the code happened to drop into that part
of the conditional.

Testing done:
Posted a change to an SVN working directory for just a single file.

Reviewed at https://reviews.reviewboard.org/r/5726/
Merge branch 'release-0.6.x'