Add support for multiple signatures

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-01-23.10:26:05.098>
labels = ['3.7', 'type-feature', 'library']
title = 'Add support of multiple signatures'
updated_at = <Date 2017-01-23.17:51:01.471>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2017-01-23.17:51:01.471>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-01-23.10:26:05.098>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 29350
keywords = []
message_count = 3.0
messages = ['286068', '286101', '286104']
nosy_count = 4.0
nosy_names = ['rhettinger', 'larry', 'serhiy.storchaka', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue29350'
versions = ['Python 3.7']