Add hidden setting (AlternateMouseScroll) for arrow keys on mouse scroll

The hidden defaults setting AlternateMouseScroll turns on "mouse wheel
scrolling support when a terminal is in MOUSE_REPORTING_NONE mouse mode and
is using an alternate screen by translating them to arrow up/down terminfo
escape sequences. (ala MouseTerm)"

Activate it with
$ defaults write com.googlecode.iterm2 AlternateMouseScroll -bool true

This patch was submitted and adapted in
https://code.google.com/p/iterm2/issues/detail?id=974
and then wrapped in a hidden setting by me
Expose isAlternate as a method of the PTYTextViewDataSource protocol and other improvements as suggested by @gnachman
Merge branch 'alternate_mousescroll' of https://github.com/FiloSottile/iTerm2 into FiloSottile-alternate_mousescroll
Minor improvements to pull request 164 for style