Add NSDate.dateFromISO8601String helper method

Adapted from soffes/sstoolkit:
https://github.com/soffes/sstoolkit/blob/master/SSToolkit/NSDate%2BSSToolkitAdditions.m

Added support for ignoring milliseconds.
Support ISO8601 server times

Add support for ISO8601 server times as defined by the server-time CAP
spec: http://ircv3.atheme.org/extensions/server-time-3.2

Attempt to maintain backwards compatibility with old server-time formats
supported in various ZNC releases (including @t and @time keys, and
ISO8601 and unix timestamp values).

Send both the old znc.in/server-time and znc.in/server-time-iso
capabilities since we now support both formats.
Merge pull request #169 from blt04/server-time-iso

Support ISO8601 server times