Use timezone marker in timestamp, fix extra bug.

utcnow is not timezone aware so isoformat will not include a timestamp
marker. use strftime to create timestmap with timezone marker.
extra can be None when using logging.config.fileConfig, handle that.
Remove some extra shitespace.
Use fmt to specify source_host and extra

When using logging.config.fileConfig (and maybe also dictConfig) it's
not possible to specify parameters to the formatter. Since
logstash_formatter isn't using the fmt/format parameter we can (ab)use
it to specify source_host and extra. This enables us to make use of
source_and and extra in fileConfig. fmt is just a json string. For
example:

[formatter_logstash]
class=logstash_formatter.LogstashFormatter
format={"source_host": "test.example.com", "extra": {"a": "123", "x":
"abcd"}}
Merge pull request #3 from romabysen/master

Use timezone marker in timestamp, fix extra bug.
allow dateformat to be supplied.
bump
removes unnecessary sys import.
implements logstash v1 formatting
updates version to 0.5.7
Update __init__.py

new line at end of file
Merge pull request #7 from joekiller/master

Implements LogstashFormatterV1
Update docstring to reflect change in arguments

The LogstashFormatter arguments changed recently but the docstring did
not change with them.
Merge pull request #9 from alphagov/fix-docstring

Update docstring to reflect change in arguments
Update README and constructor docstring
Add support for default `@fields`
Merge pull request #12 from alphagov/add-default-fields

Add support for default `@fields`
Merge pull request #11 from alphagov/update-constructor-docs

Update README and constructor docstring
Upstream merge.