Let Syntastic know we have a checker for OpenCL

This simply adds 'opencl' to the g:syntastic_extra_filetypes global
variables on plugin load. This should let Syntastic know that it has
a syntax checker for OpenCL files, and should treat them as a supported
file.

Nothing breaks if we don't do the above, but doing it adds some extra
niceness, like tab completion in Syntastic.
Add Apple's openclc compiler as a syntax checker

Previously, the only Syntastic OpenCL syntax checker included in this
package relied upon the clcc OpenCL compiler to check the file for
errors.

This commit adds an additional syntax checker for OpenCL, which uses
Apple's openclc compiler (built into OS X) as a compiler to check for
errors.

The openclc checker should only be enabled when it detects the openclc
compiler is present on the machine (though it can be controlled by
letting the variable `g:syntastic_opencl_checkers` be equal to a list
of
syntax checkers you want to use for opencl files, inside your .vimrc.)

The syntax checker will use openclc to compile the file, and by default
uses OpenCL version 1.1 as the standard, with every warning enabled
(-Weverything). These compiler arguments can be overrode by the user,
however. The syntax checker then provides error format strings to parse
openclc's output for the message, line and column numbers, and message
type.

You can manually tell Syntastic to check your OpenCL file for errors
with the `:SyntasticCheck` command. The errors/warnings/info will then
be highlighted, or the full list of them can be viewed with the
`:Errors` command.
Update comments & whitespace of openclc checker

This commit updates the comments of the openclc.vim file defining the
openclc-based syntax checker to include more information on each of the
compiler flags being used. It also strips trailing whitespace in the
same file.

No change to the code is contained in this commit; only formatting and
comments.