Add a coverge script to run tests and produce coverage reports

This PR adds a `coverage` script to run tests and produce coverage reports.

## How to use

```sh
$ atoum/scripts/coverage.php -h
# ...
-fmt <xml|clover|html|treemap>, --format <xml|clover|html|treemap>: Coverage report format
-o <path/to/file/or/directory>, --output <path/to/file/or/directory>: Coverage report output path

# Default format is XML (Clover)
$ atoum/scripts/coverage.php -f my/test.php --output coverage.xml
# HTML report
$ atoum/scripts/coverage.php -f my/test.php --output coverage/ --format html
# Treemap report
$ atoum/scripts/coverage.php -f my/test.php --output treemap/ --format treemap
```

This could be useful to ease integration with external tools (#245, Jenkins, ...).