added CMake code to find MassiveThreads and removed hard-coded path
added new configuration option EXAFMM_ENABLE_MT to switch MassiveThreads
fixed all target to use MYTH_INCLUDE_DIRS and MYTH_LIBRARIES
bugfix: fixed misspelling in CMakeLists.txt (SIMD_FLAGS-> SIMD_FLAG)
Merge pull request #6 from keisukefukuda/fix-cmake-find-massivethreads

Fixed CMakeLists.txt to find MassiveThreads and remove hard-coded linking path.