Add size() method to BatchStatement class.

Since BatchStatement#getStatements() returns an immutable copy,
and that is probably a good design decision, then lets add a
size() method so that we don't need to create a copy by calling
batch.getStatements().size() in order to get the current number
of statements in the batch.
Merge pull request #220 from mspiegel/batchstatement-size

Add size() method to BatchStatement class.