Support a virtical line tentatively
Add id column to vrules table
Add GF::Data to instance variables of GF::RRD
Change end point of posting to vrule

prepare to implement vrule summary
Support retrieve vrule definitions API

GET /vrule/summary/service/section/graph
Support vrule_legend parameter in query string
Do eval in calling get_vrule in summarize_vrule
Merge pull request #56 from hirose31/feature/virtical-line

[WIP] Support a virtical line