Allow env DSL to take a block

In addition to

  env :userpaths
  env :std

formulae and requirements can now do

  env do
    append 'PATH', '/some/path/to/bin'
    # and more
  end
Object#instance_exec for Ruby 1.8.6

Not thread safe! But I don't think we care.

We want to evaluate the env DSL block in the context of ENV for asthetic
reasons, but we also want access to methods on the requirement instance.
We can use #instance_exec to pass the requirement itself into the block:

  class Foo < Requirement
    env do |req|
      append 'PATH', req.some_path
    end

    def some_path
      which 'something'
    end
  end

Also add a simplified version of Object#instance_exec for Ruby 1.8.6.
Block-style env DSL examples
Allow satisfied? to be specified in a block

Instead of overriding #satisfied?, Requirement subclasses can specify
the condition in a block:

  satisfied do
    some_condition?
  end

The contents of the block are evaluated in the context of the instance,
and so have access to instance variables and instance methods as before.
Additionally, it is wrapped in an ENV.with_build_environment block. This
can be disabled by passing :env => false to satisfied:

  satisfied :env => false do
    some_condition?
  end
tests: add mocha mocking library
env :userpaths applies to satisfied block

When requirements specify 'env :userpaths', it is now applied to the
evaluation of the block passed to satisfied as well as the build of the
dependent formula.

It can also be specified in the options hash passed to satisfied:

  satisfied :userpaths => true do
    # ...
  end