XML-ize linebreaks and issue urls in history

Change Modification.cs to write out raw XML for comments and issueURL so
that can preprocess linebreaks and issues urls
Change history parser to xml escape the input (note only for SVN, others
will be unsafe)
RegExIssueTracker handle multiple internal links

Customise RegExIssueTracker to handle multiple internal links.
Allow multiple issue urls per comment line

Change so that support multiple different IssueTrackerUrlBuilder
instances in one comment line
(note only implemented for RegExIssueTrackerUrlBuilder)
Ignore the ModificationDelay if forced build

Ignore the ModificationDelay if the user has forced the build.