Enable testing with IBM DB2 on Travis 

added improvement type into pull request template

1. Use the actual dsdriver distribution
2. Introduced `AbstractPlatform::getCharMaxLength()` since `CHAR` and `VARCHAR` limitations are different on DB2.

Workaround for the inability to use a post-PHPUnit script on ContinuousPHP

AppVeyor tweaks, retry coverage upload on failure

Add GitHub issue and PR templates

Allow to specify arbitrary SQL expression in AbstractPlatform::getDummySelectSQL()

Fixed inconsistency in calculating date difference between platforms

Postgres, Oracle and SQL Server Sequence increment_by & min_value as int

Upgrading docs

Fixes #3121

Conflicts:
	docs/en/index.rst

For testing purposes and not only, it may be needed to evaluate an arbitrary SQL expression (see https://github.com/doctrine/dbal/pull/3013, https://github.com/doctrine/dbal/pull/3108). Instead of doing `str_replace()` on an expected string, one could specify the expression explicitly.

Replaced ArrayIterator with StatementIterator in Portability\Connection

The SQLite and Oracle platforms produces a `ROUND()`'ed or `TRUNC()`'ated difference between the datetimes. However it should be the difference between dates.

Fixes https://github.com/doctrine/dbal/issues/3114

CI: Aggregate coverage from multiple jobs

Added badges for the develop branch in README

Removed hard-coded configuration filenames from the test runner

Add sidebar.rst and cleanup docs homepage.

Consistent PHPUnit configuration across environments

Add warning about MySQL JSON key sorting

See discussion at https://github.com/doctrine/doctrine2/issues/7187

Use newer PHPUnit to prevent crashes on failures