pithcat: Remove default option values

Handle better possible backend options and require necessary
ones by printing appropriate error messages.
Fix typos in error messages and docs

Replace all references of `environmental variable' with `environment
variable'
Bump version 0.15rc3

Update ChangeLog and version files
Merge branch 'release-0.15' into debian-release-0.15
Bump version to 0.15~rc3-1~wheezy

Signed-off-by: Nikos Skalkotos <skalkoto@grnet.gr>