Add ParallelTests::Cucumber::FailuresLogger to log cucumber failures.

This class subclasess Cucumber::Formatter::Rerun and overrides the 'after_feature' method in order to lock the output file so that multiple processes can write to it.

The output format is similar to cucumber's own rerun output format so it can be re-run with cucumber in the same way. (i.e. by passing the filename to cucumber prefixed with '@', ex: `cucumber @parallel_cucumber_failures.log`).
Refactor ParallelTests::Cucumber::RuntimeLogger to use the new Io module
Update the failure logger to log each cuke failure on a separate line.

(Having each failure on a separate line gives a better indication of the amount of failures and is nicer to read)
Add a spec for the ParallelTests::Cucumber::FailuresLogger.
Update Readme.txt with usage for the cucumber failures logger
Merge pull request #202 from CDEI/add_cucumber_failures_logger

Added ParallelTests::Cucumber::FailuresLogger to log cucumber failures.