Fix UNIX time parsing (value passed by JSON.NET is long)
DateTime optimization

There's two kinds of optimization in this commit.

*	Common UNIX epoch object to make efficient and easy to read.
*	Use `Ticks' rather than `Seconds' in floating point values.
Merge pull request #67 from a4lg/unix-time-fix

Fix UNIX time parsing (value passed by JSON.NET is long)