Make executable
Remove unused imports
Remove trailing whitespace
Add optional parallel processing support

By default, this changes nothing. But "--jobs" enables the builds to be
run in that many parallel jobs. If "--jobs" is set to zero, the number
of jobs is chosen automatically based on number of CPU cores.
Avoid mutating environment variables

Instead, pass environment variables to check_call(). This allows us to
support running properly with multiple threads.
Undo some unnecessary changes
Create separate virtualenv for each configuration

Suggested in #5 regarding running concurrent build but not only.
Fix indentation
Merge branch 'parallel'

Conflicts:
	travis_solo.py
Fix #7: Parsing env incorrectly
Fix shlex.split unicode issue
Improve code formatting
Bump version