Make sure to not hit open file descriptor limit

[vojtajina]

The default limit on Moutain Lion is 256 and sometimes people get EMFILE error.

Let's make sure we are efficient.
Preprocessors should use in-memory cache.
Web server might do in-memory caching as well, considering number of captured browsers (eg. if there is only one browser, in-memory caching does not make any sense).

[vojtajina]

The preprocessor is already using fs-graceful instead of fs. Let's do these:

• globally replace fs with fs-graceful
• change preprocessors to cache content (that will save at least 2 fs access per processed file; will also enable virtual/generated files)