Invert sense of italic check

We're currently using a hard-coded list of terminals that are known to
either support or not support italics, and iTerm.app is currently in the
unsupported list.

The current HEAD of the iTerm 2 master branch, however, does add
optional support for italics:

  https://code.google.com/p/iterm2/issues/detail?id=391
  https://github.com/gnachman/iTerm2/commit/a2a7a8bfe050b03f2

This commit makes it possible for people running a sufficiently recent
build of iTerm 2 to opt in to using italics in Solarized by doing
`let g:solarized_italic=1` in their ~/.vimrc before the `:color
solarized` call. For everyone else, the behavior should be as it was
before.

Note that even if we move iTerm.app into the italic-supporting
whitelist, the ability to do an explicit opt-in in the ~/.vimrc is
especially useful in remote environments (eg. ssh'd into another host),
as the TERM_PROGRAM variable that Solarized is checking may not be
carried across into the remote environment, at least not with the
default ssh configuration for SendEnv and AcceptEnv.