Support sending tags from the log4j client.
Switch to having tags on one line.

Having multiple line property value in property files isn't very nice and although this rules out the possibility of have a comma in the tags that shouldn't be a showstopper for most people.
Added note about rebuilding tags.
For nicer code use Guava.

Guava is already a dependency of raven so we don't create any more problems with dependencies.
Switch to : as key/value separator in properties.

As = is already used in .properties file use colon to make it more readable.
Merge pull request #84 from buckett/log4j-tags

Support sending additional tags with log4j events.