Add qpdf in bootstrap.

Fixes #54.
print system information (deliberately run from R and not Rscript)
after_install -> before_script
add self-test
add tests for installation of apt and GitHub packages
add test to assert that packages aren't installed by default
install another package for testing
install binary R packages with just one apt-get install call -- again

- otherwise the logs just look horrible: https://travis-ci.org/yihui/tikzDevice/builds/13447667, lines 365-700
- runtime penalty is substantial, too
Merge pull request #55 from krlmlr/sysinfo

before running test, print system information
check pre-condition for GitHub package test
Merge pull request #57 from krlmlr/selftest

add tests for installation of apt and GitHub packages
Make a few tweaks to `.travis.yml` tests.
Make a test quieter.
Merge pull request #59 from krlmlr/binary-install-single

install binary R packages with just one apt-get install call -- again
Tweak binary name generation.
Drop a section from the sample.

[ci skip]
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
expose EnsureDevtools again through old command devtools_install

Reason: Running extensive integration tests (such as in tikzDevice) that are not part of R CMD check but require installation of the package

Installation can be done easily by `devtools:::install('.')`
Merge pull request #62 from krlmlr/keep-devtools

Expose EnsureDevtools again through old command devtools_install
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).