Allow strings for levels in the monolog service provider

The monolog service provider requires a constant on Monolog\Logger to be
referenced, which has an integer value. This is not very practical when
working with external configuration files.

This patch accepts strings like debug, info, error, etc. that are
translated to the corresponding logger constant values.

This is an alternative to #882.
Backport static:: to classname:: for PHP 5.3 compatibility
feature #885 Allow strings for levels in the monolog service provider (igorw)

This PR was merged into the 1.2.x-dev branch.

Discussion
----------

Allow strings for levels in the monolog service provider

The monolog service provider requires a constant on Monolog\Logger to be
referenced, which has an integer value. This is not very practical when
working with external configuration files.

This patch accepts strings like debug, info, error, etc. that are
translated to the corresponding logger constant values.

This is an alternative to #882.

Commits
-------

6c97e67 Fix JSON syntax error in monolog provider docs
a7cba30 Reword the documentation on string logger levels
2ef826c Backport static:: to classname:: for PHP 5.3 compatibility
121f3f2 Allow strings for levels in the monolog service provider