Write image to file to avoid failure when session size is limited

[Ryan J Ollos]

Incorporate the patch by ebencheung. As original noted on the wiki page: created a new branch WriteImgToFile: So it will work when apache's session size is limited, while the origin code will show nothing if you create large or many diagrams on a single wiki page.

​https://github.com/ebencheung/TracPlantUmlPlugin/tree/WriteImgToFile

[Ryan J Ollos]

(In [11255]) Refs #9770: Re-implemented the patch by ebencheung, with minor changes. The images are now stored in a cache directory, rather than in the session data. ebencheung had described the issue in which session data was limited by Apache, and the graphs failed to render. Caching files to disk seems like a better approach and this is similar to how GraphvizPlugin works. Some code review will be done in the future to compare this implementation with that of the GraphvizPlugin.

Minor changes from the original patch are:

• The cache directory can be specified in trac.ini. The default is $TRACENV/cache/plantuml.
• A _read_img_from_file method was added.
• Minor coding style changes.

The biggest concern I have with this implementation is that the cache directory is never cleaned. I'm not sure of the best way to handle this, but as a first step I'll take a look at if/how the GraphvizPlugin handles this issue.

[Ryan J Ollos]

(In [11256]) Refs #9770: As noted in #3951, an option for the cache directory is probably overkill, so it has been removed. It will be reimplemented if someone provides a reasonable use-case.

Genshi is now used to build the html that is returned by the macro.

[Ryan J Ollos]

GraphvizPlugin has a _clean_cache method that says like it cleans up the cache based on file access time. Something similar should be implemented here.

[Ryan J Ollos]

Added ebencheung as a contributor on the project wiki page.