Return bucket from CreateBucket() functions.

This commit changes the API for:

    Tx.CreateBucket()
    Tx.CreateBucketIfNotExists()
    Bucket.CreateBucket()
    Bucket.CreateBucketIfNotExists()

These functions now return the *Bucket and error instead of just the error.
Merge pull request #130 from benbjohnson/create-bucket-api

Return bucket from CreateBucket() functions.