Add sensors and readings tables to store DO and Temp

We need to store sensor readings such as Disolved Oxygen and
Temperature.

Create models to store those:

* sensors: one row for each metric, and maybe someday gpio info
* readings: one row per data point for a particular sensor, maybe
  someday with pre-aggregation

Add Reading#sensor to attr_accessible so that it can be mass-assigned
and user-controlled.
Add presence and numericality validations for sensor readings

Sensor reading models don't have any validations, which means the
code using them will have to be vigilant of empty values and/or
invalid data.

Add some basic validations to lighten the burden on client code.
Add unique constraint on sensors.name

We'll want to manage sensors by name, which could be confusing if more
than one sensor exists with the same name.

Enforce uniqueness of sensor names, both at the level of active record
validations and in the database with a uniqueness constraint.
Add unique constraint on readings sensor_id+measured_at

It wouldn't make sense to have two readings for the same sensor at the
same time, but the model doesn't protect against that.

Add a unique constraint on readings (sensor_id, measured_at).  It will
offer useful protection, and it will likely support many of the queries
that we'll do when reporting on this data.
Generate a Sensor scaffolds and remove unneeded parts

We're gonna need more machinery around sensor readings.  It would
be ideal to have it conform to Rails conventions as much as possible.

Generate the scaffold as a starting point, minus the already-existing
model and model specs, and remove bits we don't need, keeping the
controller, views, stylesheet, and route.
Add script to generate Temperature and Dissolved Oxygen data

We some sample data to demonstrate that graphs are displaying
appropriately.

Generate time series for two sensors: Temperature and Dissolved Oxygen.
Use a remotely realistic model to produce data that looks real enough to
imagine what really-real data would look like.  Avoid including in
db/seeds.rb because it's really more for development than a new app
install.
Chart sensor readings with NVD3 on sensors#show

We've got lots of sensor readings, and nothing conveys trends and
anomalies in a time series like a pretty graph.

Show a graph of sensor readings over time on the sensors#show page.  Use
NVD3, a library that builds charts based on D3.  Convert the show
template to slim for easy coding, but keep the chart in an ERB partial
for precise control over a little inline JS.  Keep the JS inline so we
can easily inject customized axis configurations in the future.
Configure Slim to output pretty HTML rather than space-stripped HTML to
improve debuggability of markup.
Complete the show/edit/destory workflows for sensors

We can see sensors, but managing them isn't pretty.

Convert the scaffolding to bootstrap and slim.  Customize it slightly,
for example by burying the destory button and spacing it away from other
actions.
Show current status of readings on sensor index

The sensor index shows its attributes.  But we need a way to convey
the current state of the system.

Add the current sensor value and trend to the sensor index page.

It would be nice to have a more full-featured dashboard here, for
example by showing wheather a measurements is in-bounds, by showing
the trend over the last N measurements, showing the current
value as the average of the last N measurements, and/or showing charts
of all of the sensor readings.  But let's start simple.
Add working Sensor and Feeder tabs to site navigation

The site navigation is just a visual place holder.

Add a Sensor tab, and make the Sensor and Feeder tabs work in the sense
that they activate an de-activate when clicked, and they link to the
correct pages.
Add README with installation and roadmap sections