Add a method for casting into byte string or unicode depending on python version
Fix sys.argv to reflect default Python behaviour

By default, calling the Python REPL without arguments causes
sys.argv to be a non-empty list with an empty string, and some
modules (e.g., pyNN) rely on that.
Translate all foo? -> is_foo. Close #334

The fancypants Hy award goes to Nick for coming up with the quoted
symbol hack for exports. This broke with foo?, since the export string
needs to be is_foo, but using a quoted string will pick up the change
due to it being a Symbol.

Mad clown love for that, @olasd.
Merge branch 'string-cast' of https://github.com/Willyfrog/hy into Willyfrog-string-cast
Merge branch 'master' of https://github.com/hcarvalhoalves/hy into hcarvalhoalves-master
Merge branch 'paultag/feature/support-question-marks' of https://github.com/paultag/hy into paultag-paultag/feature/support-question-marks

Conflicts:
	hy/core/language.hy