Revert "“forgiving” parsing of spurious backslashes, with directions for later"

This reverts commit 524054502d5f43c3a3f044c25d8a3e83842eb00e.

As per ongoing discussion in https://github.com/fruux/sabre-vobject/issues/129
we do it differently.
Introduce new OPTION_LIBICAL_COMPATIBLE option

Fixes #129
eh, why are the definitions in two places?
add a testcase, and fix a bug spotted by it ☺ and add a changelog entry