(GEP-126) Align at-expression arguments as literal list

This commit adds a layout for the AtExpression that will format
the [] part the same way a literal list is formatted (and thus
controlled by the same style preference settings).
(GEP-179) Handle dropped support for node inheritance

Node inheritance is no longer supported in Puppet versions >= 4.0.
This commit ensures that Geppetto will warn about this deprecation
in older versions (can be ignored by changing default preference
setting). Versions >= 4.0 will always report errors.
Merge pull request #42 from puppetlabs/issue-179

(GEP-179) Handle dropped support for node inheritance
Merge pull request #41 from puppetlabs/issue-126

(GEP-126) Align at-expression arguments as literal list
(GEP-171) Support the * => syntax in resource expression

Ensure that '*' can be used as an attribute name, but only if
target platform >= 4.0. Also check that LHS of attribute is either
dynamic or a literal hash.
(GEP-172) Support resource body with arbitrary rvals in title

Puppet versions >= 4.0 will allow using arbitrary rvalue expressions
as the title in resource bodies. This commit ensures that Geppetto
doesn't complain about such expressions unless the target platfrom
is < 4.0.