Using Set() for the empty set instead of Set([]).
Using let instead of var where possible.
Added tests for Set(…).
Added Set usage examples.
Removed import from usage example.
Merge branch 'master' of github-as-practicalswift:practicalswift/Pythonic.swift
Added another Set test case.
Updated documentation.
Minor cleanups.
Added file.close().
Moved the sequence-related function to its own extension.
Merge branch 'master' of https://github.com/practicalswift/Pythonic.swift

# By practicalswift
# Via practicalswift
* 'master' of https://github.com/practicalswift/Pythonic.swift:
  Minor cleanups.
  Updated documentation.
  Added another Set test case.
  Removed import from usage example.
  Added Set usage examples.
  Added tests for Set(…).
  Using let instead of var where possible.
  Using Set() for the empty set instead of Set([]).