Moved delay/period to first argument of post_in/post_every

[geky]

per suggestion from @kilogram, and I believe @pan- had similar concerns with the initial design

• makes the call clearer, especially when callback argument looks like a delay value
• simplifies implementation of c++11 wrappers

matches previous change in c api (https://github.com/geky/events/issues/3, https://github.com/geky/events/pull/4)

[geky]

I'm going to go ahead and merge this.

Note: this is a compatiblity breaking change. v0.2 marks the previous version.