Add docs for new charts features
configure must be re-run with all previous options as well

Otherwise defaults such as database type and database user may be
rewritten.
rt-test-dependencies doesn't look at $DisableGD, it needs --with-gd
Clarify that out of the box RT will handle Unicode in charts

Link to the %ChartFont option while I'm at it.
Minor word choice tweaks for clarity
Using July 1 and 31 is more clear, at least to me

Noting TIMTOWTDI is ancillary to showing people how to use charts.
Minor typo and wording fix
mention devel package required to build perl's GD

It's required on some systems, but on very popular.
Point to docs/charts.pod for GD install instructions
Merge branch '4.2/charts-docs'