Add package:vendor_gems tasks

Prior to this commit, the packaging repo had no way of vendoring gems to
be included in a source tarball. This commit add such a task so that
prior to running `rake package:tar` a package builder can run `rake
package:vendor_gems` and the necessary gems for the project will be
pulled down and stored in `vendor/cache`.

NOTE: Later steps in the build process will be required for making use
of these cached gems. Those later steps might look something like:
```
bundle install --local --deployment
rm -rf vendor/cache
```
Merge remote-tracking branch 'jeff/add_vendor_gems_task' into feature/master/pre_tasks
Add support for pre_tar_tasks

This commit pulls in PR 118 from @jeffweiss
(https://github.com/puppetlabs/packaging/pull/118) which adds a tasks that
puppet dashboard will need to execute prior to composing its tarball. The
pre-tar task execution can actually be quite useful for other projects. So this
commit adds a simple conditional to check for the existence of the pre_tar_task
attribute in the build object (which will be added by the build_defaults or
project_data yaml files), and then executes the task prior to composing the
tarball.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Document pre_tar_task and other flags in README.md

This commit adds a quick explanation to the README of the pre_tar_task as well
as for some other flags we failed to document - build_doc, build_ips, and
build_pe.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Make sure bundle exists before calling it
Merge pull request #122 from MosesMendoza/feature/master/pre_tasks

Feature/master/pre tasks