Handle ticker symbols with characters that produce invalid R object name

`getSymbols` strips the leading caret (`"^"`) from index ticker symbols in order to return an object with a valid R name. This [StackOverflow question](http://stackoverflow.com/questions/14760622/access-odd-named-object-returned-by-getsymbols) illustrates another case that needs to be handled.

Read the getSymbols entry point and the linked Stack Overflow example first. Reproduce the invalid-name ticker case, then define completion as getSymbols returning a usable R object name for that ticker without breaking the existing leading-caret handling.

Written by the indexing model from the issue text.