Support incremental builds using BuildContext, fixes #6

I followed the instructions at the following URL to support incremental builds: http://wiki.eclipse.org/M2E_compatible_maven_plugins 

This required changing to using BuildContext to link to files and to scan if there were changes since the last incremental builds.

I also updated my Eclipse to 4.3.2, and updated M2E to 1.5, after which the test project I am using sesame-vocab-builder with, https://github.com/podd/podd-ontologies (branch named sesame-vocab-builder for the integration), works in Eclipse and displays the generated-sources as a source folder.

I was using Eclipse-4.3.0 and M2E-1.4 before that, so I am not sure which change would have been more beneficial, but most likely updating M2E rather than Eclipse.

I also added guessing for mime-types based on the URL or file name, using Rio.getParserFormatForFileName, to enable users to avoid entering both the global and the file/url-specific mimetype parameters if they are using any of the extensions recognised by Rio, based on the parsers on their classpath.