Exportcert command updates [mid v1.4]

[sharan8]

This pull request closely follows the improvements outlined in #175.

The following updates have been implemented:

• Refactor to use predicate classes EventContainsEventIdPredicate and RecordContainsVolunteerIdPredicate for filtering through the event records and events.
• Exported certificate design is updated with better text alignments, text sizes and templated text.
• Tests are updated to reflect exporting for volunteers with records only.
• Support for absent event records is abstracted away from createPdf method, and is now placed earlier, in the execute method itself.
• Added TypicalVolunteersWithRecords.java to be able to run tests on a model which has both volunteers as well as records. (Note: exportcert only works on volunteers who have records)
• Updated tests to use both model with records and model without records for testing
• Hardcoded separator '/' replaced with File.separator to be OS agnostic.
• Certificate export folder renamed to 'Certs'
• 'INDEX' added to 'Certificate exported for volunteer at INDEX ...' to be more user friendly

Area(s) of Concern:

• Could the inclusion of addRecordForVolunteer() in CommandTestUtil to add an event record for the first volunteer be improved?

[coveralls]

Coverage increased (+0.8%) to 71.594% when pulling db6a0e9 on sharan8:exportcert-command into f77c7fd on CS2103-AY1819S1-W16-2:master.

[iMarbles]

Remember to change tag to 'Ready' once you're done!