Fix for readline REPL problem Darwin Python3

On OS X Mavericks, the Hy REPL doesn't allow the 'b' character when
running with Python 3.3. This appears to be a common problem
for Darwin and readline as mentioned in this post

http://superuser.com/questions/297527/cant-type-the-b-letter-in-python-shell-in-os-x
Make _storeize generic

This allows to change an expression's context to something else than ast.Store if needed.
Implement del

Closes #385.