Add in a new core language file.

 This will let us implement common functions seen in other lisps,
 and allow them to be importable, without explicit imports. The goal
 is to keep this as small as we can; we don't want too much magic.

 I've added `take' and `drop' as examples of what we can do.
documented else / finally syntax for try / catch
Merge pull request #202 from paultag/paultag/feature/start-language-lib

Add in a language lib.
Get Hy working on Windows by making readline use conditional.
Merge branch 'master' into pr/213
Style fixes.
Updating NEWS entries.
Move code from hy.readline_helpers into new hy.completer.completion context
manager so we can "wrap" readline completion around REPL interaction.
documentation for some of the builtins
references #18
Style fixes.
Merge branch 'master' into pr/222