Add some basic validations

This adds validations for the following:

 * `Calendar#title.length`: between 2 and 140 characters
 * `Calendar#description.length`: between 10 and 3000 characters
 * `Event#title.length`: between 2 and 140 characters
 * `Event#description.length`: between 10 and 3000 characters

I also added some validation tests using shoulda matchers.
Let me know what you think in terms of the limitations.

ref #27