Ruyaml

- fixing issue 54 empty lines with spaces (reported by Alex Harvey)

- fixing issue 46 empty lines between top-level keys were gobbled (but
not between sequence elements, nor between keys in netsted mappings
(reported by Alex Harvey)

- fixing issue 45 preserving datetime formatting (submitted by altuin)
Several formatting parameters are preserved with some normalisation:
- preserve 'T', 't' is replaced by 'T', multiple spaces between date
and time reduced to one.
- optional space before timezone is removed
- still using microseconds, but now rounded (.1234567 -> .123457)
- Z/-5/+01:00 preserved

- Fix for issue 44: missing preserve_quotes keyword argument (reported
by M. Crusoe)

- correct 'in' operation for merged CommentedMaps in round-trip mode
(implementation inspired by J.Ngo, but original not working for merges)
- iteration over round-trip loaded mappings, that contain merges. Also
keys(), items(), values() (Py3/Py2) and iterkeys(), iteritems(),
itervalues(), viewkeys(), viewitems(), viewvalues() (Py2)
- reuse of anchor name now generates warning, not an error. Round-tripping such
anchors works correctly. This inherited PyYAML issue was brought to attention
by G. Coddut (and was long standing https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=515634)
suppressing the warning::

import warnings
from ruamel.yaml.error import ReusedAnchorWarning
warnings.simplefilter("ignore", ReusedAnchorWarning)

- minor improvements based on feedback from M. Crusoe
https://bitbucket.org/ruamel/yaml/issues/42/