add dependencies for Homebrew Formulae

new DSL key `depends_on_formula`
update new ios7-screensaver Cask
- fix checksum
- convert caveats to do block
- use manual_installer from the caveats DSL to generate standard msg
  with full path
- use undocumented caskroom_only feature to make clear what is going
  on here, pending the addition of screensaver artifact support
always submit external commands in list form

For safety.
- This is a step toward reworking system_command.rb so that
invocations are done in list form on the back end, avoiding
surprises from quoting and shell metacharacters.
- There is one transitional hack here: the _quote method in
Cask::SystemCommand is modified to avoid double-quoting.  The
_quote method itself will go away in a future revision when
only list-forms are used.
- Casks using system are not touched. It seems natural to
address that when creating the DSL for after_install/before_install.
Added Skreenics, a video thumbnailing tool.
Added Praat, a speech analysis software package.
Merge pull request #2603 from stylerw/skreenics

Add Skreenics Cask
Merge pull request #2604 from stylerw/praat

Add Cask for Praat, a speech analysis software package.
Added Psychopy 1.79.01

Sourceforge URL (latest) according to guide downloads windows version.  URL used here functions and DLs mac version. Sorry about the audit warning, let me know if this needs to change.
Add Sage 6.0.

Sage is a free open-source mathematics software system licensed under
the GPL. It combines the power of many existing open-source packages
into a common Python-based interface.
Update Adobe Reader homepage
Merge pull request #2608 from leoj3n/adobe-reader-homepage

Update Adobe Reader homepage
Merge pull request #2595 from rolandwalker/list_args_to_system

always submit external commands in list form
don't use commas in artifact stanzas

- no functional changes in this patch
- preparation for interface change allowing custom
  target links in #2532
Correct Bowtie link

Fixes #2609
tests for #2532

Travis won't like this until/if #2532 is merged.
Added cask for xact 2.24

Enjoy!
Merge pull request #2610 from stylerw/xact

Added cask for xact 2.24
Added 7zX 1.7.1

Wish I could have used a cleaner DL url, but the official site uses a magnet link (lol).  So, MacUpdate it is.  It works, though!
Merge pull request #2611 from stylerw/sevenzx

Added 7zX 1.7.1
Allow customizing the application symlink