Add Mushroom, Titanic datasets

[ocramz]

Notes:

• Both Mushroom and Titanic are for supervised binary classification tasks
• Both have categorical-valued features
• Titanic rows have 3 features, Mushroom rows have 22

[ocramz]

Ping @glutamate : )

[ocramz]

@glutamate the Travis tests are failing for a reason unrelated to this PR; is there something I should change or fix here?

[glutamate]

Apologies, I thought I merged this some time ago. Don't worry about the CI, it often breaks due to out of memory errors

[ocramz]

No worries, glad you liked it! I can't wait to try it out in a project, and I hope it'll be useful to others as well : )

[ocramz]

@glutamate could you upload a new Hackage release? Also, I'd be happy to help co-maintain this package if it's ok with you.