Support case-insensitive usage

Refers to #43

Sample usage:

import fire

class Example(object):
  def alpha(self):
    return 'alpha'
  def Alpha(self):
    return 'Alpha'
  def Beta(self):
    return 'Beta'

if __name__=='__main__':
    fire.Fire(Example)

$ python test.py alpha
alpha

$ python test.py Alpha
Alpha

$ python test.py beta
Beta

$ python test.py BETA
NOTE: Consider using the correct capitalization for BETA
ERROR: Ambiguous member access: BETA
Usage: test.py <command>
  available commands:    Alpha | Beta | alpha

For detailed information on this command, run:
  test.py --help

google / python-fire

Support case-insensitive usage #426