Protocol-based loading of templates does not work

[oschettler]

When using a protocol other than "file://" to load templates with Mustache_Loader_FilesystemLoader, I get an error FilesystemLoader baseDir must be a directory.

I have provided a pull request to fix the code and test.

I've fixed the code to check for existing directory and file only when there is no protocol.
To make the test Mustache_Test_Loader_FilesystemLoaderTest.testConstructorWithProtocol actually fail for these checks, I have implemented a minimal protocol "test://".