(fact-273) Return a string for stub of :kernel fact

Previous to this change, this spec test was stubbing :kernel
as :windows. But now facter a) does type-checking on fact
values (Symbol is not allowed) and b) warns on type-check
errors.

Changing this stub to return 'windows' reduces warning
messages during spec runs.
(fact-273) Expect a warning message that can be expected

Add a warning message expectation to suppress the warning message
on stdout/stderr during spec runs.  Also, remove a stub that
duplicated a stub in the before block.
(maint) Don't warn on non-existent paths

Facter was emitting warnings whenever a path specified by
`Facter.search_path` was not a directory, but Puppet was automatically
adding two paths via the `factpath` setting that seem to never exist.
This could cause hundreds of warnings to be generated, none of which are
really that useful. This commit changes the warnings to only be emitted
when a manually specified path is not absolute and silently ignores all
paths that aren't directories.
(fact-273) Remove a test which was a subset of the following test
(fact-273) Add expectation for Facter.warn on timeouts
(fact-273) Add expectation for Facter.warn on exceptions retrieving operatingsystemrelease
(fact-273) Add expectation for Facter.warn on ignored file extensions
(doc) Update documentation for Facter#search
Merge pull request #626 from adrienthebo/maint/facter-2/fewer-warnings-from-search-path

(maint) Don't warn on non-existent paths
(maint) Update loader specs to use let blocks over ivars
(fact-273) Add expectation for Facter.warn on resolutions that throw an exception
Merge branch 'maint/facter-2/cleanup-loader-specs' into facter-2
Merge branch 'facter-2'

Conflicts:
	lib/facter/util/loader.rb
	spec/unit/util/loader_spec.rb
Merge pull request #627 from kylog/issue/fact-273/address-warnings-in-spec-runs

Issue/fact 273/address warnings in spec runs
Merge branch 'facter-2'