WIP: PSR-3 compatible Logger

[tvdijen]

This checks some of the boxes in #371:

• Extract the log levels definition from \SimpleSAML\Logger and use the ones defined by Psr\Log\LogLevel instead.
• Add a $context parameter to all the methods defined by Psr\Log\LoggerInterface.
• Make the \SimpleSAML\Logger class extend Psr\Log\AbstractLogger.
• Make the \SimpleSAML\Logger::log() method public and make it throw a \Psr\Log\InvalidArgumentException if the log level is unknown.

[codecov]

Codecov Report

Merging #1438 (bfaeb14) into master (7a1d8e6) will decrease coverage by 6.96%.
The diff coverage is 33.33%.

❗ Current head bfaeb14 differs from pull request most recent head aa21c42. Consider uploading reports for the commit aa21c42 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1438      +/-   ##
============================================
- Coverage     45.48%   38.53%   -6.96%     
+ Complexity     3692     3482     -210     
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  Files           158      139      -19     
  Lines         12292    10373    -1919     
============================================
- Hits           5591     3997    -1594     
+ Misses         6701     6376     -325