Feature information fix

This PR fixes #7 by adding the [Squeel gem](https://github.com/ernie/squeel) to allow for a nice DSL for constructing queries instead of using `find_by_sql` directly. This should mean that the queries will work with any underlying SQL database. Obviously, as this is a fix, it works with Postgres now!

For example the new query in the Squeel format is :
```
  _start = start
  _experiment_id = experiment_id
  _reference_id  = reference_id
  Feature.where{|f|
    (f.experiment_id == _experiment_id) & (f.reference_id == _reference_id) &
    (f.start >> (_start..stop)) & (f.end >> (_start..stop))
  }.order{|f| [f.start.asc, f.end.desc]}
```