development updates
 - skeleton for Charlson score calc
 - braces around some short functions in explain.R for consistency
 - parse.R half-way through updating to use RTF from 2011/12, not 09/10 for the master ICD9-CM list. WIP for #2
 - valid now contains Filter functions that can be magrittr-style piped through, and tests
 - short to major E code tests
 - tests for issue #2 : extracting the top level icd9 codes correctly from an RTF
 - slightly simplified mark-up in vignette.