Simplefix

Fix a few issues pointed out by LGTM.
Note support for Python 3.7.

* Added new `remove()` function to delete a field from a message
* Added new `__str__()` special function, useful for showing a message in logging or debugging.
* Linked to https://simplefix.readthedocs.io from the README, hopefully making the detailed docs more visible.
* Added more constant values from the FIX specifications.

* Added support for Python2.6 to support RHEL6/CentOS6 which doesn't EOL until November 2020.
* Added support for `in` and `not in` tests for tag numbers in messages.
* Adding a field with a value of `None` will silently fail.
* Unless it's preceded by a length field, a data type value will be treated as a standard (string) value.

Some major changes to the use of strings (vs. bytes) for Python 3.x, with all received values now exported as bytes, and input values being transformed to bytes using UTF-8 encoding (from strings) and ASCII encoding for everything else. If you want to use a different encoding, transform to bytes yourself first, but you probably should be using the FIX _DATA_ type for encoded values anyway?

Also a major expansion/rewrite of date and time value handling. Added a bunch of method covering all the FIX date/time types properly. The existing `append_time` method is deprecated, in favour of more specifically named method for UTC and local timezones, and datetime, date-only and time-only values.

Add support for adding "header" fields: they are inserted, starting at the beginning of the message, prior to any existing fields. This allows FIX header fields, for instance SendingTime(52), to be added after the body fields.

Added handling of FIX 'data' type fields to the parser. Data fields can contain arbitrary data, including the SOH character, and were not previously supported.