refactor :stderr => :silence

replace with :print_stderr => false

The double test on options[:print_stderr] in system_command.rb
is intentional, and temporary.
recast :print => true as :print_stdout => true

in Cask::SystemCommand
remove :print_stdout => false

this is the default, and need not be specified
switch to %Q{} for complex quoting
Return Cask::SystemCommand::Result object

from Cask::SystemCommand.run

* stderr and stdout are now separated, though both are available
* :print_stderr is made the default, closer to previous behavior
* exit status can be read from the result object
* plist parsing is more naturally handled in the result object.
  The :plist argument to the run method was removed.
* whitespace
Merge pull request #6329 from rolandwalker/system_command_refactor

refactor Cask::SystemCommand