Add parser tests for accepting non-keywords

[swolchok]

Resolves issue #58. There is one special case because fragments named on may be defined according to the current specification (fragment on on SomeType { someField }), but may not be referenced.

(If you try to reference your fragment named on, it's at best a syntax error, as in { ...on }, which is the start of an inline fragment definition that doesn't have a type.)

[leebyron]

Since 8ae5bd7 just landed, maybe you want to revisit this slightly to not include "on" as a viable fragment name? If you rebase, this test should no longer pass

[swolchok]

If you rebase, this test should no longer pass

It currently passes; on is a legal query name, type name, field name, directive name, argument name, and variable name, and the test is careful not to use it as a fragment name.

[leebyron]

It currently passes; on is a legal query name, type name, field name, directive name, argument name, and variable name, and the test is careful not to use it as a fragment name.

Nice work