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.
ignore non-assignments when parsing args
merge Verification with DotEnvFile
rename Environment to EnvKeyParser
refactor capistrano integration

* use formatter from cap task
* unit spec for cap task
* integration test without formatter
* exception based verification result
integration test for linted env