exportcert final updates

[sharan8]

Exported certificate is updated to include:

• Border
• Event ID
• Total hours served by the volunteer

Implementation logic is now improved to account for:

• More than 20 events overrunning the page. This is done by limiting the number of records displayed to 20 and truncating the rest.
• Long event names. They are truncated as well, to ensure that the record entry doesn't exceed the width of the certificated.

CertGenerator class is refactored to break down the pdf population into several methods, and making better use of shared class fields.

[coveralls]

Coverage increased (+0.2%) to 67.846% when pulling 255bf10 on sharan8:exportcert-command into 4886b00 on CS2103-AY1819S1-W16-2:master.