Add syntastic for syntax checking

Syntastic will run syntax checking for many different languages. 

* The MRI Ruby syntax checker runs a syntax check with warnings and tainting checks enabled. I'm not really clear on what the latter does.

* The Sass syntax checker will use the `--check` option on the `sass` binary. 

* CoffeeScript will be checked with either the `coffee` or `coffeelint` binaries if they are available in your path. Both require node (`brew install node`).

* JavaScript will be checked with JSLint or JSHint if they are available. Both require node and are installable with npm.

I have configured syntastic with the options I have been using for over a year now which are pretty close to the defaults.