Remove linking against Oracle OCIEI lib.

This should make a minimal approach to solve #89.
According to discussion on OTN, there is no reason to link against libociei directly as the exposed client interfaces are in the libclntsh.dylib file and we should only be linking against the client library. See details at https://forums.oracle.com/forums/thread.jspa?threadID=2506565

If possible, I'd prefer to keep FindOracle.cmake minimal as SOCI does not need fully-featured FindOracle.cmake from TOra. Also, TOra's module follows outdated idiom instead of canonical use of FindPackageHandleStandardArgs as explained in this thread http://www.cmake.org/pipermail/cmake/2010-March/035597.html
In future, it would be great to combine both modules and refactor according to current CMake guidelines.