re-add DotEnvFile

we need a place to store the name of the .env.example file, so we can
pass it to error objects to create helpful error messages.
move UnrecognizedDotEnvLine into errors file
env verification

verify defined env variables based on
* a hash
* the output of export

verify only known args are used
basic rake and capistrano tasks
print list of missing/unkown variables

failing at the first invalid variable is not very helpful.