DOC-1137: added always_cache_features to config_important_settings.markdown

Added to Puppet master settings, basics: the always_cache_features setting should be set to true to cache features whether or not the feature test succeeds.
DOC-1137 added link for always_cache_features

Added a link to configuration.html#always_cache_features:
DOC-1130 Added note re: deprecated node inheritance

Replaced most of the node inheritance information with a note that node inheritance is deprecated in Puppet 3.7.
DOCS-1148: resource collector can't collect classes

Added a note to lang_collectors.markdown that classes cannot be collected.
DOCUMENT-101: added `import` deprecation notice on 3.7 lang_node_definitions

We deprecated `import` in 3.6, but no deprecation notice was added.
DOCUMENT-101: added `import` deprecation notice for 3.6 lang_node_definitions

We deprecated `import` in 3.6, but forgot to put a notice on this page. Now we have done so! This commit is for the 3.6 docs.
DOCS 1140: edited backslash requirement in Windows package provider `source` attribute

Added info to resources_packages_windows for 3.7 that forward- and backslashes are accepted by the `source` attribute for Windows package provider

Added info to resources_package_windows for 3.6 that only backslashes are acceptable in `source`, as this information was missing from the page.

Changed lang_windows_file_paths for 3.6 to correctly reflect the `source` backslash requirement.
DOCS-101 further changes

Edited lang_node_definitions page for brevity re: `import` deprecation, in both 3.6 and 3.7 docs

Clarified always_cache_features setting to be more user-friendly and helpful.
Merge pull request #372 from jbondpdx/puppet37

Puppet37