Use microsecond timestamps for thrift inserts.

Previously, same-ms inserts were using the same timestamp which
means the first insert might "win" over the second. Using a new
dependency "microtime", default timestamps now have microsecond
precision which makes things a thousand times better.

The Column class now has a timestamp_micro field you can use if
you want to inspect the high resolution timestamp since javascript
Date objects only support millisecond precision.
Make thrift remove use microsecond timestamps.

Like insert, remove now uses microsecond precision timestamps
by default (you can still pass in a Date object).
Move microtime to optionalDependencies.

This way if you have trouble installing/building microtime on Windows, you
can still use helenus (albeit without microsecond precision timestamps).
Merge pull request #123 from nearbuy/micro_time

Use microsecond precision time for insert/remove