Add files.upload support as file_upload() into pyslack

[joshuakwan]

Just added some code to support the files.upload method. Not able to write a successful mock test so I attach some test code and result here:

from pyslack import SlackClient

slack = SlackClient('xxxxxxx')
res = slack.file_upload(['#test-everything'],'event_handler.py', filetype='py')
print res

{u'ok': True, u'file': {u'filetype': u'python', u'lines_more': 0, u'channels': [u'C077NK40J'], u'display_as_bot': False, u'id': u'F08V40GFR', u'size': 369, u'url_download': u'https://slack-files.com/files-pub/T04D85BG2-F08V40GFR-0ce8bd5b7a/download/event_handler.py', u'title': u'event handler', u'url_private': u'https://files.slack.com/files-pri/T04D85BG2-F08V40GFR/event_handler.py', ...}}