add datetime as version scheme

[tschmidtb51]

I suggest to add a version scheme datetime that is defined as follows:
To compare 2 versions a and b, do:

Compare the as datetime parsed values (including the timezones).

Use case: Especially in cloud environments, often the version is given by a certain timestamp.

Flagging @pombredanne for attention

[prabhu]

Software should never use datetime as versions since different machines could have different time and timezone set (skews). What is wrong with semver?