add BOOSTRAP_LATEX option to bootstrap command to install TeXlive on Ubuntu

- LaTeX is installed if BOOTSTRAP_LATEX environment variable is set
    - Ubuntu: use list of packages provided by Dirk
    - OS X: install basic version of MacTeX package (BasicTeX)
        - full version cannot be installed due to insufficient disk space (travis-ci/travis-ci#1586)
        - use explicit Utah mirror recommended by MacTeX site, seems to be fastest
        - remove package files after installation

- add test for BOOSTRAP_LATEX option in build matrix
Make mac+latex easier.

This makes two tweaks to Kirill's hard work:
* Add a base set of tex packages, so that we can use this for testing out of
the box.
* Change the way we handle paths, so that users don't have to source anything
in their `.travis.yml` unless they're doing something fancy (like installing
more of their own packages).
Merge branch 'kirill-merge'
Make a number of small (mostly cosmetic) tweaks:

* Pick a `[[` style
* Set up parallel tex/no-tex tests
* Move config options to the wiki
* Kill a stale comment or two
* Add `set -x` in `travis-tool.sh`
Make Dirk's suggested edits to build/check defaults.

Fixes #64.
Closes #65.
scripts to maintain parallel OS X branch
make .travis.yml work unchanged on OS X
fix check for clean working copy
don't ever bump, just overwrite
hybrid
Merge pull request #60 from krlmlr/osx

scripts to maintain parallel OS X branch
Merge remote-tracking branch 'parentrepo/master'