moved the IO availability test into the prover and checker records
add usability attribute of external tools to xml logic graph
use usable provers and cons-checkers
isLeft is missin in Data.Either of ghc-7.6.3
added reasons for missing provers, etc
also filter for usable conservativity checkers
added service for available provers #1378
also adjust the displayed logic grpah to usable provers
corrected binary name
Merge pull request #1381 from spechub/1377-all_provers

1377 all provers