Pytest-cpp

- Properly handle fixture failures in Boost.Test.
Thanks fj128 for the PR.

- Use XML in CAPS since beginning at Boost 1.61 the parameter value is case sensitive (29).
Thanks edisongustavo for the PR.

- Proper fix for 25, the fix made in `0.4.1` was incorrect.

- Fix error that may happen during collection when using xdist (25).

- Integrated most of the examples found in boost-1.55 for Boost Test into the
`pytest-cpp` test suite. This uncovered a problem when Fatal Errors were
produced and which would break py.test.

- Integrated all the official Google Test examples into the `pytest-cpp` test
suite, ensuring that Google Test is fully covered.

- Fixed 17: supporting Type-Parametrized tests in Google Test. Thanks
joarbe for the report.

- Now any executable passed explicitly to py.test in the
command-line will run as a test, regardless of the `cpp_files` option.

- Now capturing standard error while collecting tests, otherwise
executable files that are not test suites could write into `stderr`,
which would mess with `py.test`'s output to console.