For operations that read all formulae, catch exceptions and
skip broken ones, rather than bomb out entirely.

A user's PATH can help diagnose ./configure and other problems in bug reports.

When a formula specifies both an `url` and a `head`, and the latter
specified a `:using` spec, brew would try to fetch the `url` using the
specified download strategy.  With this change, brew respects `:using`
specs only when determining the download strategy for `head`.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

* Add tests that run only if 'jruby' is installed.
* Note that if your formula has :jruby deps, it should likely
  "depend_on 'jruby'" as well.

If submodules exists for a git repo then checkout their index as well
into the appropriate path.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

A 'head' in a formula can now specify which download strategy to use via
a ':using' specification:

    head 'http://svn.macosforge.org/repository/darwinbuild/trunk/',
        :using => :svn

This reduces the number of cases where "download_strategy" needs to be
overriden.