Write `Time` Chapter

[runpaint]

The Time class needs discussing. Doesn't fit the scope of any existing chapter.

[runpaint]

Outline:

• Instantiation: Time.at, Time.utc/Time.gm, Time.local/Time.mktime, Time.new/Time.now
• Arithmetic: Time#+, Time#-, Time#<=>, Time#succ
• Predicates: Time#dst?/Time#isdst, Time#friday?, Time#gmt?, Time#monday?, Time#saturday?, Time#sunday?, Time#thursday?, Time#tuesday?, Time#utc?, Time#wednesday?
• Attributes: Time#day, Time#gmt_offset/Time#utc_offset, Time#hour, Time#mday/Time#day, Time#min, Time#mon/Time#month, Time#nsec/Time#tv_nsec, Time#sec, Time#to_a, Time#tv_sec/Time#to_i, Time#tv_usec/Time#usec, Time#wday, Time#yday, Time#year, Time#zone, Time#to_f Zone Conversions: Time#getgm/Time#getutc, Time#getlocal, Time#gmtime, Time#localtime, Formatting: Time#asctime/Time#ctime, Time#to_s, Time#strftime