Fixed json support

Fixed PlantUML json support when the json is defined in an included file or variable.

Auto-create cachedir if missing

Another minor improvement/fix: the path indicated in 'cachedir' is now automatically created if it doesn't exist.

Speed improvement when generating PNG images

This version fixes a small issue that affected performance when generating PNG images from diagrams without hyperlinks.

Converted as a Python module

The extension has been refactored as a Python module to simplify installation.

Sall fix for the package release

This version has a small fix in the release script to be able to load the source archive into PyPi as well.

Fixed corrupted inline svg images

Sometimes inline SVG images were edited by Markdown who treated them as normal text.

Fixed urllib3 warning about use of insecure connections

When using a PlantUML server with self-signed certificates it is necessary to set the insecure configuration option to true, allowing for insecure connections.
But in this case 'urllib3' gives us a warning that it will appear on the rendered page; this release resolves this issue.
It also solves another small problem, when using other types of diagrams (such as plantgantt) with a PlantUML server.

Added two new configuration options

Added the following configuration options:

• config: set the PlantUML configuration file (as the -config plantuml command line option)
• plantuml_cmd: set the plantuml command to execute (defaults to plantuml); can be used for passing custom options to java, for example java -Dplantuml.include.path=includes -jar plantuml.jar.

See the README.md for more details.

Changed value for the insecure setting

The default value for the insecure setting, which disables the check of SSL certificates, has been changed from "False" to False (its now a boolean value).
This does not change the behavior of the settings, but it's more compliant with the session.request option.
See #86 more more details.

Forced UTF-8 encoding

Forced UTF-8 encoding when files are passed to local plantuml.
Make sure that the diagram files are in utf-8, especially in Windows.