Add formula for include-what-you-use

Builds against clang 3.5. See [their homepage][1] for more information.

Was first attempted in #24783. I decided to just use the svn repo since that gets updated more frequently than downloads.

[1]: https://code.google.com/p/include-what-you-use/
I mean, I'll change it. It's just:

  a. Weird style choice
  b. `brew audit` didn't complain (it didn't complain about my lack of hashrockets either. What is the point of `brew audit`?)