Replicate the entire environment when executing compass.

When using RVM, it's important that we replicate our runtime environment
for subprocesses. Making a calls to System.getenv ensures we do that.
Let's not explicitly call the String constructor.

In other news, I need to learn to double check things before I write
responses to comments on Pull Requests.
Better building of the new process environment.

Changes as per code review.
Merge pull request #12 from farmdawgnation/environmental-protection

java.lang.Runtime.exec overwrites environment.

Replicate the current process environment + add the
explicit asset_domain setting when invoking compass.