Rename chart attribute to chart_file

The @chart attribute in Job was holding the value of the file to write
out the chart data to, which is a poor name. Changed this to chart_file
to match the conventions of json_file and csv_file, and to be overall a
better description. Also remove reader for this attribute.
Add Chart class

Create a Chart class to hold chart-related knowledge. At this point it
is just a basic stub, and just the basic chart writing code from Job is
copied over into a generate method. Job creates a Chart and passes a
reference to Report and the sizes object. Further refactoring needed.
Rename chart_data method to just data

Especially since this is the object that the JSON and CSV writers are
using to output the data.
Cutting out unneeded chart_for method
Rename chart_hash(label) to data_for(label)
Move method that generates chart options into chart class
Move creation of comparison data into chart model
Only pass report data to chart

Now that all the chart-specific methods are pulled into the Chart
object, we don't need to be passing the entire report object into the
chart. Instead, we just pass the representation of the report data.
Don't need to pass sizes around anymore
Cleaning up variables in Chart

Too many things had the name chart in them, too many things were named
opts, and generally too many long variable names and duplicated code.
Cleaned a bunch of that up.
Minor name cleanup. Freeze data sent to chart to prevent unwanted manipulation
Pull out logic of how to build comparison data sets into separate method
Splitting out the data generation code into methods to calculate the scaling factor and methods to generate the particular data sample for a size n
Switch comparison types back to symbols and create mapping of symbol to chart label. Move types into array to be able to iterate generation code. Calculate sample size in initializer as it can be calculated from @sizes. Then don't pass sample_size everywhere
axis variable is not necessary
Generate readable title from type symbol
Switch array example step_size back to 1000
Add constant line to comparison charts
Merge pull request #9 from davy/refactor-charts

Refactor charts