Update BUILD.md

Add OSX build instructions taken from https://gist.github.com/number5/7250865
Merge branch 'develop' of github.com:MultiMC/MultiMC5 into feature_yggdrasil
Could reach end of rulesFromJsonV4 method without returning a QList

It was possible for execution to reach the end of the rulesFromJsonV4
method without returning. This was causing a crash on OS X when parsing
rules inside an instance.
Merge https://github.com/Drayshak/MultiMC5 into develop
Merge https://github.com/brainiac744/MultiMC5 into develop
Merge branch 'develop' of https://github.com/Drayshak/MultiMC5 into feature_yggdrasil
Merge branch 'develop' of github.com:MultiMC/MultiMC5 into feature_yggdrasil
Add icon for when no default account is selected

Also fixed a bug where the icon on the accounts button wouldn't change
when the user checked the "Use as default" checkbox in the account
selection dialog.
Tweak the layout of the accounts button
Verify access tokens before launching Minecraft

Kind of an important thing to do... Heh...
Rework the password dialog

It's now used as a general purpose "account edit dialog". It'll be used
for entering usernames, passwords, or both.
Nuke dead code
Fixed compilation errors.

Yeah... Forgot to recompile everything after deleting those files.
Fix OS X packaging problem

GLOB_RECURSE was pointing at the wrong directory - it needed to point
at the directories that had the dylib files in them to fix them so they
didn’t point at the local installed version of Qt
Remember the last selected instance and select it on startup
Fix crash when no instances are available
Merge https://github.com/brainiac744/MultiMC5 into develop
Fix login and startup logging issues

Auth uses the refresh endpoint instead of validate. This means less password entering.
Console will now only autoscroll when already scrolled all the way down.
Better conformance with the Yggdrasil auth protocol (not complete yet, but Mojang launcher isn't complete either).
Fix bug that prevented saving the account data (uninitialized variable).
Accounts can now trigger account list saving, this is used for the refresh endpoint.
Fix private info censor
Fix (hopefully) library dependency resolution.

Installing libs is now enabled, hardcoded.
Enable -Wall for all builds.
Fix many warnings and latent bugs.