revamped roslaunch $(find) stuff

 * modified roslaunch $(find PKG) to consider path after it for resolve strategy
 * add separate commands:
   * $(find-executable PKG PATH)
   * $(find-share PKG PATH)
   * $(find-share-devel PKG)
   * $(find-share-source PKG)

Update - the following commands have been removed again:
 * $(find-share-devel PKG)
 * $(find-share-source PKG)

Update - the following command has been renamed:
 * $(find-share PKG PATH) -> $(find-resource PKG PATH)

Update:
 * Since #233 has been merged which only affects the $(find) command, I have updated this pull request to only contain:
   * $(find-executable PKG PATH)
   * $(find-resource PKG PATH)