enable 'Find Implicit Dependencies' to make Tests run in Xcode

This fixes the random linker errors you see when doing Product -> Test
from Xcode.

Since 'Parallelize Build' was enabled, the order in which Xcode was
building the targets was non-deterministic.  Sometimes dependencies
would get built in the right order, but sometimes not.  The implicit
dependencies stuff sorts this out.
'make test' should specify the iphonesimulator SDK for running tests.

Without this, we'll get the project default which is 'iphoneos' for ARM,
and then we won't be able to run the tests.
Merge pull request #32 from fpotter/fix-make-test

this should get `make test` working...