Parse lcov with branch summaries but no branches.

Parse an `lcov.info` file that contains `BRF` and `BRH` records but no `BRDA` records. To support this, each `item` object is created with a `branches` property. The `branches` property is not only created if needed. To determine if there are no branches in an `item` object, instead of testing for the absence of the `branches` property, the user should instead to the `found` property of the `branches` property.

See #3.