Add additional attributes to CRF object for better compatibility with sklearn interface

[SethPoulsen]

Fixes #60

[MeMartijn]

Hi there!
Because I ran into this same issue I decided to install sklearn-crfsuite using your branch as a source (via pip install --upgrade git+https://github.com/SethPoulsen/sklearn-crfsuite.git#egg=sklearn_crfsuite). However, it does not seem to solve the issue for me, I still get the AttributeError from #60. Would you be able to share your scikit-learn version that made this fix work for you?

[SethPoulsen]

Your command looks like you installed from the default branch of my fork, can you try installing from the add-missing-attributes branch, which is where I made the changes?

[MeMartijn]

Whoops, my bad, from the specific branch it works, thanks for the fix!

For anyone that doesn't want to wait before this is merged, you can install a fixed version like this:
pip install git+https://github.com/SethPoulsen/sklearn-crfsuite.git@add-missing-attributes#egg=sklearn_crfsuite

[MeMartijn]

I needed this change and the change from #67 for my code to work, so, seeing that the package is not updated anymore, I created a new fork with your fix and the fix from #67.

If anyone needs both, you can start using an updated version of this package by running this:
pip install git+https://github.com/MeMartijn/updated-sklearn-crfsuite.git#egg=sklearn_crfsuite. Any updates are welcome, I'll accept all pull requests for bug fixes.