clarified tiddler timestamp assertion

previous pattern was misleading
removed empty fields from text serialization

these provide no value and only lead to confusion
added creator field to text serialization

the fact this was previously missing is largely due to historical
reasons, as creator wasn't part of the original design and the text
serialization was created with the text store in mind

however, serializations should be generic, so this moves store-specific
serialization adjustments into the respective store (unfortunately, the
implementation within the store seems a bit awkward though)

note that this change not only affects HTTP responses, but also
subclasses
reduced noise in test output

not sure if @cdent will like this though
replaced ternary for readability
Merge branch 'text-serialization-refactor' into full-text-serialization
moved tiddler fields into class attribute on text serialization
Merge branch 'text-serialization-refactor' into full-text-serialization

Conflicts:
	tiddlyweb/serializations/text.py
reused tiddler model's fields listing for text serialization

@cdent will probably hate this big time, but duplicating model
attributes all over the place seems undesirable

note that I did not adjust the JSON serialization accordingly because
things seem subtly different there (whitelisting a select set of
attributes when deserializing)
Merge branch 'text-serialization-refactor' into full-text-serialization

Conflicts:
	tiddlyweb/serializations/text.py
restored empty fields in text serialization

... while omitting them in the text store's representations - this
required introducing non-standard arguments, thus the text store now
accessed `tiddler_as` directly rather than through the `Serializer`
interface
Merge branch 'text-serialization-refactor' into full-text-serialization

note that this makes some tests fail - which is expected though; code
adjustments to follow

Conflicts:
	test/test_serialize_tiddler.py
	tiddlyweb/stores/text.py
added omit_fields option to text serialization

this allows the text store to omit the creator field, which is
calculated rather than persisted in that context

cf. 99a3d574f0b478546bde56994c39d4a9324caa12
disambiguated Tiddler attribute
Merge branch 'text-serialization-refactor' into full-text-serialization

Conflicts:
	tiddlyweb/serializations/text.py
removed obsolete import
Merge pull request #60 from FND/full-text-serialization

add creator field to text serialization