Fix MySQL case-insensitive search, order by id when looking for next short code

[Hu1-Li]

1. fix bug when getting latest short code using order by link.id
2. fix bug when comparing two short code using collate

[coveralls]

Coverage increased (+0.1%) to 53.917% when pulling 7185d7c on Hu1-Li:issue15 into 863fc7a on amitt001:master.

[amitt001]

Sorry for not getting back early. Please make comment changes, squash the commits in one and rebase the branch with the master when done.

[amitt001]

I am sure it would be a good idea to write migrations here. This will clutter the code.

Instead of doing alter table we after the table is created one solution is to do

if config.database['engine'] == 'mysql':
    short_code_data_type = Unicode(6, collation="utf8_bin")
else:
    short_code_data_type = Unicode(6)

short_code = Column(short_code_data_type, unique=True, index=True, default=None)

This handles MySql case. As postgresql and sqliet are already case sensitive.

[amitt001]

This will also require moving DatabaseFactory import in initialize.py to initialize_db function.

[Hu1-Li]

This is my first time working with CI, I think i can do better next time, sorry for the inconvenient.

As for merging these commits into one commit, you can select squash and merge in here

[amitt001]

Fixes #15

Squashed your commits and fixed with commit 67260ba

@Hu1-Li your initial event.listen way of altering in case of MySQL was better as checking config.database['engine'] == 'mysql' in Link class made it unusable for directly importing and using it. It became dependent on config. Sorry for all the rework.

Thanks for fixing this.