compiler: add `else' support in `try'

This is a bit tricky, since we'll also have to support `finally' in the end,
I've introduced an Else statement on my own to be able to recognize it.

In the end, we even do a better job than Python (sort of) since we support a
syntax that doesn't in Python:

  (try
    (pass)
    (else (pass))

is translated to:

  try:
      pass
  else:
     pass

Which seems impossible in Python syntax. How fun?

This fixes issue #74.

Signed-off-by: Julien Danjou <julien@danjou.info>