Expanding distro support & adding alpine buildpack variant

[callbacknull]

This PR basically begins to add rudimentary support of distro's beyond debian and ubuntu. The first of those I thought would be a nice addition is the alpine distro

I think having support for some minimalist linux distros has some very strong advantages that the community will enjoy.

• Size - Alpine 3.3's 2MB image vs Debian Jessie's 50MB image vs Ubuntu Trusty's 66MB image
• Faster image distribution & boot time
• Better Security - Less code and programs are deployed with each container; We drastically reduce our viable attack surface.

Size comparison:

buildpack-deps           trusty-curl         199.6 MB
buildpack-deps           trusty-scm        279.6 MB
buildpack-deps           trusty                532.6 MB
buildpack-deps           jessie-curl        169.4 MB
buildpack-deps           jessie-scm       296.7 MB
buildpack-deps           jessie               622.3 MB
buildpack-deps           3.3-curl            10.68 MB
buildpack-deps           3.3-scm           111.5 MB
buildpack-deps           3.3                   376.8 MB

Issues or possible improvements:

• I have the version as 3.3 right now, which might be a little ambiguous. I can make this alpine-3.3 and parse it in the update.sh. Thoughts?
• I think I found equivalent packages for almost everything. I couldn't find any magick* packages or a libcurl4-openssl-dev - There is a curl-dev which also installs openssl-dev. Also, mysql-client was replaced with mariadb-cllient in alpine 3.2

[callbacknull]

Build is failing because of caching issues that's reported in #40 which is unrelated to the integrity of this PR

[tianon]

Interesting, although IMO the goals of using alpine and the goals of using buildpack-deps are somewhat at odds with one another. The point of buildpack-deps is essentially to trade disk space for "successfully compiles arbitrary external code" (ie, any gem, pip, npm install), where if you're using alpine you're going to want to start lean and stay lean (since it really isn't that difficult to do the pip installs you need and figure out which packages you're missing -- it's just a PITA for someone who's brand new to Docker to try and figure out).

[callbacknull]

@tianon yeah okay I see your point. :) I also realize my grand idea of making a pr to docker node to use this as a base wouldn't actually work.