Support input data formats other plain/text

[sodre]

According to GATE documentation:

The jobs are run using GATE Embedded version 8.3, and your application must use GATE plugins that are compatible with this version. The following plugins are pre-loaded by default in order to support additional input data formats: Format_Twitter, Format_MediaWiki, Format_PubMed, Format_FastInfoset and Format_DataSift.

This change entails adding the plugin dependencies to build.gradle, and finding a way to add the mime-type before we open the document.

[sodre]

Gate uses a plugin system to detect Document formats. We use the Content-Type passed by the browser/client to the document class.
Here are the ones that are pre-installed:

mime-type	Documentation
text/plain
text/html	HTML
text/xml	XML
text/json	GateJSON
application/fastinfoset	FastInfoset
text/x-cochrane	Cochrane
text/x-pubmed	PubMed
text/x-mediawiki	MediaWiki
text/x-json-twitter	Twitter
text/x-json-datasift	DataSift