Add coach to Roster

[roclark]

The coach or manager's full name is now a property that can be retrieved from a team's Roster class.

Closes #285

[codecov]

Codecov Report

Merging #527 (b47b34b) into master (8383487) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #527   +/-   ##
=======================================
  Coverage   99.97%   99.97%           
=======================================
  Files          55       55           
  Lines       13108    13168   +60     
=======================================
+ Hits        13105    13165   +60     
  Misses          3        3           

Impacted Files	Coverage Δ
sportsreference/mlb/constants.py	100.00% <ø> (ø)
sportsreference/nba/constants.py	100.00% <ø> (ø)
sportsreference/nba/nba_utils.py	100.00% <ø> (ø)
sportsreference/ncaab/constants.py	100.00% <ø> (ø)
sportsreference/ncaaf/constants.py	100.00% <ø> (ø)
sportsreference/nfl/constants.py	100.00% <ø> (ø)
sportsreference/nhl/constants.py	100.00% <ø> (ø)
sportsreference/mlb/roster.py	100.00% <100.00%> (ø)
sportsreference/nba/roster.py	100.00% <100.00%> (ø)
sportsreference/ncaab/roster.py	100.00% <100.00%> (ø)
... and 3 more

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