Add ExternalProject reference to Google Test 1.7.0.
Partially resolve google/benchmark#17 by fixing regular expression leak.

This adds a unit test to validate the wrapper without running the entirety of
benchmark_test.
Resolve memory leak from benchmark instances in google/benchmark#17.

This shifts registration from a global vector to a singleton that manages
benchmark destruction during shutdown.
Remove URL_HASH and TLS_VERIFY from CMake configuration.

Per the CMake 2.8.0 documentation, these options did not exist for
ExternalProject_Add.  These options were added in CMake 2.8.10.