parseEnv ignores variables with no value

[jlogsdon]

This is inconsistent with the way shells handle them. I am working on a project that requires some variables to be set to bogus values in development, and I generally like to define it as a "no value" variable. This works if I source .powenv and start up the rack application manually, but parseEnv tosses out any variable without a value due to the regex /([^=]+)=(.+)/.

The obvious quick-fix is to put actual data into the variables, but I think the environment parsing should be consistent with other tools. I spent the last 30 minutes trying to figure out why this wasn't working properly.

[jeremy]

Moved to #380