Add option to TeamCity hook to just check for pending changes

@tackley @theefer

 *This needs review because my Ruby-fu is weak and I haven't yet managed to even get the Rake test suite running on my Ubuntu box...*

The default behaviour of 'Always running a build' (with the `add2Queue` parameter) is not always desirable, because some TeamCity setups can have carefully configured rules about when to _actually_ do a build which are ignored by this direct approach. This commit adds the option
to just poke the TeamCity instance to check for changes (effectively the same as hitting the 'Check for pending changes' option in the TeamCity web user interface), which allows any TC build-rules to be applied.

The `checkForChangesBuildType` parameter is not well documented, but it is directly referenced by a JetBrains employee Yegor Yarko here:

http://youtrack.jetbrains.com/issue/TW-4134#comment=27-9255