Adding several functions to interact with environment variables.

In addition to removing the superfluous 'isNullString' function,
now that I understand the FFI a bit better, there is a new
'getEnvPair' function that looks into the 'const char** environ'.
Setting, unsetting, and listing all of the environment variables is
now supported.
Merge pull request #491 from gabemc/feature/environment

Adding several functions to interact with environment variables.