user expects to see an assessment chart on requirements#index

This fixes #13. This pull request adds a visual cue of requirements compliance for a given user by means of Lazy High Charts. The commit adds a lot of code and complexity which is needed to avoid using ugly class methods in requirement class which always require a user as arguments. By means of a service class (RequirementSeries, deserves a better class name but I couldn't find better) to which I provide the Requirement class and a user I build the data series of complete/incomplete requirements percentage and labels ("Complete"/"Incomplete") for HighCharts to build the pie chart.

Complete/incomplete code to helps building the data series for the high_charts helper. To simplify I created a service class called RequirementSeries which receives the Requirement class and a user as attributes. Leveraging on user<->proofs<->requirements relationships I use the user proofs count for a given requirement as a base to calculate the percentage of complete requirements, since a user can submit only 1 proof per requirement. To calculate the number and percentage of incomplete requirements I subtract the number of complete requirements from the total number of requirements.

A simple capybara specs checks for the existence of a div having a data attribute created by HighCharts in requirements#index view. I then unit tested Requirement model to ensure the data series where correctly build. I'll add unit tests of the service object instead of a Requirement model spec in the future. 

* adds lazy_high_charts gem and poltergeist driver for capybara
* calculate incomplete/complete requirements