Implement a build artifact archiving mechanism

[jawher]

This feature relies on the following configuration keys:

• archive: takes a single or a list of globs. The matching files are archived regardless of the result of the build
• archive_success: takes a single or a list of globs. The matching files are archived in case of a successfull build
• archive_failure: takes a single or a list of globs. The matching files are archived in case of a failed build

The archived files are then exposed over http using the following endpoint:

bzk_server:3000/variant/{id}/artifacts/{path}

Where id is the variant id and path the path to an archived artifact.

Fixes #144

[jawher]

An example config for a java project:

language: java
jdk:
  - oraclejdk8
  - openjdk8
  - oraclejdk7
archive:
  - target/classes
archive_success: target/*.jar
archive_failure: 
  - target/surefire-reports

[jawher]

Just a note: the approach taken here, i.e. storing and serving the build artifacts from the filesystem, will be a problem for #71: distributed build since this pins a particular build to a particular machine.