Source block strips escape characters

[Clorith]

This makes the syntax highlighting incorrect, as well as mis-representing the quality of our code base.

See for example ​http://developer.wordpress.org/reference/functions/_wp_specialchars/ Line 8 where an escape in the regex is stripped out.

[SergeyBiryukov]

Related: #512

[coffee2code]

In 722:

Code Reference: Pass source code through addslashes() to preserve slashes when stored in postmeta. Fixes #524