Add optional war build triggered with -Pwar ( See #419 )

I wanted to test deploying Sagan as a WAR to tomcat. To do this I added an option to the build.gradle, where if the gradle property -Pwar=true is specified, then the following command would build a WAR rather than a JAR file.
```
./gradlew :sagan-site:clean :sagan-site:build -x check -Pwar=true
```
If -Pwar is not present, then the Jar will be built as normal.

StaticPagePathFinder was also modified as when converting to a war the leading '/' was being trimmed from Resource Url paths. Tested this in tomcat on both Linux Centos & Windows 7.

A gradle property seemed like the best way to make the WAR optional. Let me know if it should be done a different way.

Also, ran into an error when trying to convert issue 419 to a pull request. 
```
C:\...\sagan>hub pull-request -i 419
...
Error creating pull request: Unprocessable Entity (HTTP 422)
Invalid value for "issue": "419"
```
Not sure what I am missing, so I am submitting the pull request manually through Github.