Check if jenkins_home/plugins exists

The code was raising an exception if ~jenkins
existed, but ~jenkins/plugins didn't. Change
the code to check for plugins from jenkins_home.
replace str2bool with validate_bool
lint fix
Add note about plugin dependencies to README.markdown
Merge remote-tracking branch 'upstream/master'
Merge pull request #83 from jchristi/master

Add note about plugin dependencies to README.markdown
Merge pull request #82 from dcrissman/dev

Lint Fix
Merge pull request #68 from ambakshi/fix-jenkins-plugins-facter

Check if jenkins_home/plugins exists
confine fact to only run on linux boxes

Confining this fact required moving the conditional and variables inside the `setcode` block, however the overall logic of the fact is unchanged. This change also includes the recent patch 7c03e35ce4f5d2c7a112142cf58cb714f95dff02

I've tested this on Linux Jenkins box, and on a Windows box and it's working as expected.
Merge pull request #87 from robbyt/patch-1

confine fact to only run on linux boxes
fix typo in init.pp
Merge pull request #88 from UCB-RAC/master

fix typo in init.pp
Change whitespace, indentation or line-breaks
Change variables in template to puppet 3.x way
Merge pull request #71 from kronn/resolve-puppet-warnings

Resolve puppet warnings
Support setting the slave mode
Merge pull request #95 from wleese/master

Support setting the slave mode
use the proxy to download the plugins

if $jenkins::proxy_host is set then we should use
it to do the wget of the plugins.
puppet will set default mode if nothing is set. Since jenkins needs to write in its home and plugin_dir - mode needs to be set to allow this
Merge pull request #99 from KlavsKlavsen/fixdirperms

set necessary mode on jenkins folders