Compatibility Module: introduce helpers

This module aims to help users of Hourglass to transpose date and time
from the time module.
Fix a spelling and add a comment in Compat module
Compat Module: stylistic change
Compat Module: move let(s) into a where clause
Compat Example: use a Patter Match instead of accessors

This is to highlights the type used in the example and how to convert
them into hourglass types
Merge pull request #10 from vincenthz/compat

Compatibility Module: introduce helpers