Import fields in logship.py 

Running govuk_logpipe with the -f argument currently fails:

$ govuk_logpipe  -f host=foo.bar.com application=mongodb
Traceback (most recent call last):
  File "/usr/local/bin/logship", line 9, in <module>
    load_entry_point('tagalog==0.2.3', 'console_scripts', 'logship')()
  File "/usr/local/lib/python2.6/dist-packages/tagalog/command/logship.py", line 41, in main
    msgs = fields(msgs, args.fields)
NameError: global name 'fields' is not defined

This is because logship.py does not currently import fields from __init__.py

This commit fixes that...