Add utcnow sqlalchemy expression

[gwax]

This PR introduces a compiled SQLAlchemy FunctionElement to provide UTC time on the server: utcnow. Server times are primarily for use as default values in UtcDateTime columns:

table = Table(
    'some_table', MetaData(),
    Column('time', UtcDateTime(), default=utcnow()))

[codecov-io]

Codecov Report

Merging #4 into master will decrease coverage by 1.31%. The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
- Coverage   96.77%   95.45%   -1.32%     
==========================================
  Files           3        4       +1     
  Lines          31       44      +13     
==========================================
+ Hits           30       42      +12     
- Misses          1        2       +1

Impacted Files	Coverage Δ
sqlalchemy_utc/version.py	100% <100%> (ø)	:arrow_up:
sqlalchemy_utc/now.py	92.3% <92.3%> (ø)

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[gwax]

@dahlia this PR is now ready for merge

[dahlia]

Thanks for your patches. This one is what you've actually wanted to merge to the master, right? Is it okay to release a new version at this time?

[gwax]

Yes, this is the feature that I have been building toward. Now would be a fine time to cut a new version.

[dahlia]

@gwax Just released a new version!