Merge pull request #10 from alvarolopez/fix_readme_and_reqs

Fix readme and requirements
Support for aggregates (metagroup+group_by).
Merge pull request #14 from orviz/add_aggregate

Support for aggregates (metagroup+group_by).
Fixed YAML report example
Use loadables for collectors

Since we are going to have several collectors in the near future we need
a mechanism to automatically load them, and execute them as appropriate
to get the defined metrics. This introduces a new list option "collectors"
that is to be set to the class names of the collectors being used.