support for user defaults (-SomeDefault=value) on command-line.

You can override some xcode settings like IDEBuildLocationStyle, or
IDECustomDerivedDataLocation by passing user defaults on the command
line to xcodebuild.  xcodebuild's man page documents this.

This add support for user default options and makes sure that xctool
doesn't barf when parsing -showBuildSettings output that includes user
defaults.

Test Plan:
Added some tests, and ran --

xctool \
  -project TestProject-Library/TestProject-Library.xcodeproj
  -scheme TestProject-Library \
  -configuration Debug \
  -sdk iphonesimulator \
  -arch i386 \
  ONLY_ACTIVE_ARCH=NO \
  -IDEBuildLocationStyle=DeterminedByTargets \
  build

... and verified that IDEBuildLocationStyle took effect and output
showed up under TestProject-Library/build instead of the normal
DerivedData location.

This close issue #57.