BrowserPlugin.php: replace HTML strings with heredocs

In preparation for the move to SabreDAV 2.1 browser plugin format, which
uses heredocs.
Config: add unit tests
Propflags: add unit tests
MD5Filter: add unit tests
Cache: Extend key to 24 bytes; 20-byte key unsupported in PHP 5.6

Our unit tests under PHP 5.6 were erroring:

  mcrypt_encrypt(): Key of size 20 not supported by this algorithm.
  Only keys of sizes 16, 24 or 32 supported

So add four bytes at the end to satisfy the alignment requirement.
Merge pull request #7 from aklomp/unittests

Unit tests, browser plugin cleanups