Show complete function signature in the pum.

Before only (fn) was displayed.
Make fn signatures in pum opt-in. Update docs.

Introduce new option: `tern_show_signature_in_pum`, which defaults to 0
meaning that (fn) will be displayed to the right of a function name. Set
the option to 1 and the function signature will de displayed, including
parameter names, their type and whether they are optional.

Update the help file to mention `tern_show_signature_in_pum`.
Merge branch 'master' of github.com:datamadsen/tern_for_vim

Conflicts:
	autoload/tern.vim