Add some development dependencies

This is just so I can get the thing running on my machine.
Create a class for toughness measurements
Create a new table for toughness measurements

It looks like when the schema was previously put together it had some extra
bits about plpgsql and btree, not sure what this is, should ask Jo.

    # These are extensions that must be enabled in order to support this database
    enable_extension "plpgsql"
Install rspec
Define some basic tests for the toughness measurement

I know that the measurement won't be valid with so little information,
I just want to have this set up in a testing framework from the start.
Create a TODO list
Create Branch and TraitsTree models
Add a Plot model

N.B. This doesn't need a database migration because it is already
defined in Jo's set of normalised tables!
Produce a valid ToughnessMeasurement
Fix the code matching regex
Tidy up logic for setting a code
Make the B in the code optional
ToughnessMeasurement can read csv
Unexpected schema change
Move code logic out to a code reader class

This class knows a lot about plot, tree and branch. As the codes start to
become more complicated I will move the logic for branch codes to Branch
for example.
Update toughness measurement to use new code reader
Generate branch architectures
Schema includes branch architectures
Update TODO
Tidy up find or create branch code