Infer the reader driver, no options required

This patch adds a StageFactory::inferReaderDriver() method that takes
only a filename, no options. The provided options were not used as part
of the inference process, so the base method should not required them.
The original inferReaderDriver(std::string, Options) has been updated to
call this new method.
Without reader type, infer from filename

Add logic in PipelineReader::parseElement_Reader to attempt to infer the
reader type from a filename provided in the options, in absence of a
explicit type.

Fixes #278.
Merge branch 'master' into issue/278-infer-reader-type

Conflicts:
	src/StageFactory.cpp
Merge pull request #336 from gadomski/issue/278-infer-reader-type

In a pipeline, infer the reader type from filename in the absence of a type attr