Add WARN level check for KERN table

[davelab6]

Observed behaviour

Ideally all our TTFs should have KERN tables, so that they typeset with kerning in older systems that don't use GPOS kerning - sadly this is still all too common.

MSFV already has a check for this, but I want something more verbose and explanatory for humans than that check result.

Before shipping that check, we should make a script that generates KERN tables from GPOS kerning in order to give people a way to solve this.

[davelab6]

http://forum.glyphsapp.com/t/old-style-kern-table-export/3322/7 explains how to get a kern table in glyphsapp and thus what an upstream GA check is for this. However, it needs some additional filtering work in the build script.