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).
Merge branch 'kirill-merge'