Support for database URLs

[dzuelke]

With a bunch of tests.

Of note:

1. in the case of information present in both URL and "normal" parameters, I'm currently giving priority to the information from the URL; IMO this makes more sense than vice versa (base info would be defined in params, and each developer or environment has a URL that may or may not override that base info, such as charset)
2. the syntax for SQLite is sqlite:///relativepath.db or sqlite://ignoredhost/relativepath.db for relative, and sqlite:////tmp/absolutepath.db or sqlite://ignoredhost//tmp/absolutepath.db for absolute paths to the database file (I went back and forth on this, but this way is easier and more consistent; https://github.com/kennethreitz/dj-database-url does the same)
3. extra query params are simply "copied" into $params verbatim; makes sense IMO especially considering point number 1
4. the URL/driver mappings may need some review

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[Ocramius]

doesn't need to be prefixed with _ anymore (was an old convention)

[Ocramius]

Also required for this PR: documentation changes in configuration.rst

[beberlei]

@dzuelke Can you add a paragraph to docs/en/reference/configuration.rst? Then its good to merge.

[kingcrunch]

The four slashes look quite strange in my eyes.

What I want to say: That looks unusual and unintuitive to me. Why don't you simply use DSN?

sqlite::memory:
sqlite:/foo/bar.db
mysql:host=localhost;port=3307;dbname=testdb

Anyway: Like to see this merged (in any form), because I find it annoying to have all this parameters for a single connection 😄

[beberlei]

@kingcrunch This is modelled after URIs that IaaS services provide on various platforms, i.e. Amazon RDS, Heroku and so on. This allows developers to use what is provided without further parsing.

[kingcrunch]

Now I could ask, why they used this format instead of DSNs, but that would go to far 😉 I'm convinced (not, that it matters somehow)

[dzuelke]

There's your docs, @beberlei and @Ocramius :)

[beberlei]

@dzuelke thanks,merged

[dzuelke]

🚀