Fix for issue #156

    1. Added DateTime.parse_to_option to utils
    2. Used this to check for valid dates
Merge branch 'master' of github.com:suaveio/suave
Revert "Fix for issue #156"

This reverts commit 8decf32f8fd261fe88f43991048627df4cb727f7.
Revert "correct fix for #156", to be replaced with 400-error return.

This reverts commit 7ac858cecff35f27d40a561993192f20c7e221ed.
Added Choice parsing for DateTime
return 400 Bad Request when invalid if-modified-since header is received
moved to invariant culture for parsing of DateTime
Simplification of Date parse handling.
Revert "Revert "correct fix for #156", to be replaced with 400-error return."

This reverts commit 2d931dfb4125d5a57de577ae6d8511b7db39fc1d.
Fixed missing 200 status introduced by 4c40144e1a42051fe7b50e7125eecd0cb1fe5a9a
Adjusted DateTime parsing to use RoundtripKind style.
updated naming of 'date_time' parse function to match convensions
Fixed usage of Parse.date_time
Merge pull request #157 from varon/master

Fix for issue #156