Add checkstyle rules

[jasta]

This is intended to run on Travis-CI to fail builds which don't adhere
to the style.

Closes #239

[jasta]

Oops, didn't work for some reason.

[Turbo87]

• the configFile should be configured using the rootDir project variable
• you should explicitly set the checkstyle version used by gradle via toolVersion (note that some Gradle versions are incompatible with same checkstyle versions. e.g. Gradle v2.5 isn't playing nice with checkstyle v6.8)

checkstyle {
    configFile = new File(rootDir, 'checkstyle.xml')
    toolVersion = '6.7'
}

[Turbo87]

also you should probably put this block into the allprojects block above to apply the plugin to all subprojects

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