Revised message severities

This PR changes message severities based on the following principles:

 * all checks that are arguable or retailer-specific are now `SUPPRESSED`
 * all checks that can be informative but do not represent a SHOULD rule
   in the spec are now `USAGE`, and the related message string reworded
   accordingly
 * in some rare cases, the severity level has been increased to match
   a SHOULD or MUST in the spec which was previously unchecked
   (e.g. `ACC_009`, `ACC_011`, `CSS_015`).

Adapted tests accordingly.

Related issues:
 * Closes #376 (we can open a new issue to discuss the feasibility of having a dedicated switch for "best practices" or "accessibility" checks.
 * Closes #379 (by making `OPF_020` `SUPPRESSED`)
 * Closes #407 (by making `ACC_003` `SUPPRESSED`: simply checking the presence of a `title` attribute is too naïve in many cases. It can be a retailer policy, but there's little sense to generally report that in EpubCheck).
 * Deals with #441 by making `ACC_014` `USAGE`, but this can be further improved by adding the `inherit` keyword to the set of allowed "relative" sizes (I'll create a separate PR).
 * Fixes #438 by making `NAV_003` `SUPPRESSED`.


@apond @santoch :

 * please tell me if you agree with the principles above, and review accordingly. We'd like to merge this ASAP to cut another alpha for EDUPUB people
 * I brutally updated tests in the `com.adobe.epubcheck.test` package by replacing "expected" JSON documents by the "actual" ones I got from a previous run. This probably makes some tests moot, so you may want to revise that by running some of these tests with overridden message severities. If you intend to, are you OK to do so in a separate PR ?