Upgrade to JRuby 1.7.13
Auto-set system ID on StreamSources where possible

For File and open-uri'd IO's we can grab the path / base_uri and set the
System ID on the StreamSource automatically, partly addressing PR #1.

Update dev dependencies and introduce VCR so we can actually use
`open-uri` correctly (it handles `file://` URLs differently from http
ones)
Allow StreamSource system ID to be set explicitly

Addresses the last of the System ID stuff in #1
Use the new SourceHelper
Pass new system_id opts stuff through the chain

Saxon::XML and Saxon::XSLT correctly transmit the options hash to the
right place, as does Saxon::Processor#XML and Saxon::Processor#XSLT
Merge pull request #2 from fidothe/improved_stream_source_handling

Improved stream source handling