Swap in dedicated .env handling library

Hi @mattn,

I hope this isn't too forward - but I've swapped out your .env handling code for a call to a dedicated .env library that I wrote.

Given that the original ruby dotenv library was taken out of foreman, and then foreman relies on it I thought it might be good to do the same thing with the Go versions of both libraries.

I've got a pretty decent set of tests at https://github.com/joho/godotenv/blob/master/godotenv_test.go so this should up the robustness of .env handling for goreman users.