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.
Correct usec -> nsec usage in Profiling.cpp and HalideProf.cpp
Add int64 cast for correct multiply results on 32-bit systems
Use linux_clock when __linux__ is defined. Also fix %d -> %lld usage.
Merge pull request #159 from steven-johnson/master

Upgrade clocks to nsec resolution