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.
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
Fixed Texmaker 4.1.1 to allow install

Previous version was unable to install ("symlink source is not there").  Added path relative to folder in zipfile to repair.
Merge pull request #2612 from stylerw/texmakerfixed

Fixed Texmaker 4.1.1 to allow install
Merge pull request #2583 from rolandwalker/no_commas_in_link

don't use commas in artifact stanzas
Merge pull request #2532 from voanhduy1512/symlinked

Allow customizing the application symlink
Merge pull request #2543 from rolandwalker/test_alt_target

tests for #2532 - customizing target symlink
doc followup for link :target

Followup for #2532.
Merge pull request #2616 from rolandwalker/doc_target_followup

doc followup for link :target
Process SystemCommand arguments in list format

Previously, SystemCommand commands and arguments were processed as
strings which caused problems during shell interpretation if the
arguments were not escaped properly.  Now all commands are expressed
as arrays and no longer require their arguments to be escaped.