add missing / to snippets/apache.yaml
separate dev_basics from basic_utils
add missing / to snippets/mysql.yml
snippets/php5.yaml: update composer installation

  - download composer.phar directly instead of curl ... | php
  - add notes to snippet description
cleanup main executable, improve error handling
Allow assets, vars, cmd, and docker.ports to be missing from definition
lib/dockergen/snippet.rb: remove variable values from Snippet

instead DockerGen::Snippet::interpret recieves a value mapping
reorganize definition file structure

- a definition file has two separate parts: Dockerfile, Makefile
- refactor code accordingly
add .gitignore
lint: private class indentation
refactor lib/dockergen: break down DockerGen submodules

- snippet handing: DockerGen::Snippet
- build directory generation: DockerGen::Job
define DockerGen errors, refs #7
introduce DockerGen::Build::Action, refs #8
major surgery, closes #1, #2, #3, #6, #7, #8, #9

shuffled things around as follows:
- DockerGen::Build::Config encapsulates all configuration
- DockerGen::CLI encapsulates the CLI frontend
- DockerGen::Build::Action generalizes actions to be taken on build context
- DockerGen::Build::Snippet encapsulates snippets which emit Actions
- DockerGen::Errors contains all DockerGen related errors