Merge pull request #156 from steven-johnson/master

Make Profiler usable on Android.
Upgrade clocks to nsec resolution

Replace halide_current_time() and halide_current_time_usec() with
halide_current_time_ns(), which returns time in nanosecond increments.

Add linux_clock.cpp, which uses clock_gettime() instead of
gettimeofday() [and actually provides ns resolution].

Have Android use linux_clock.cpp by default.