don't Fail if trying to mkdir when the dir already exists
add a script to generate large synthetic manifests

To be used by a manifest parser perf test in a follow-up.
Support both slashes on Windows when making output dirs
Merge pull request #728 from sgraham/both-slashes-on-windows-for-output

Support both slashes on Windows when making output dirs
Merge pull request #699 from mostynb/avoid_useless_mkdir_failure

don't Fail if trying to mkdir when the dir already exists
performance: Writer.build should copy less.

The build method copies the input lists several times. That
cost about 0.1s in the gyp generation for the Chromium project for
no gain.
Merge pull request #732 from bratell-at-opera/bratell/fewer_list_copies_20140414

performance: Writer.build should copy less.
Merge pull request #722 from nico/manifestgen

add a script to generate large synthetic manifests
CLParser shouldn't read stderr
Merge pull request #734 from sgraham/no-stderr

CLParser shouldn't read stderr