Feature/relative release

Add support for incremental releases as described in #15 using `slt-release -up patch`.

Internally, this includes the following enhancements:
 * make `slt info` more useful in shell scripts:
  * `slt info name` prints package name from package.json
  * `slt info version` prints version form package.json
  * `slt info repo` prints github repo name (ie. strongloop/strong-tools)
 * add `slt semver` as wrapper for `semver` command from isaacs/semver module
  * useful for scripting without adding peer deps

/cc @raymondfeng @bajtos