Add option to TeamCity hook to just check for pending changes

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

(credit to @rtyley who originally wrote this)