Bugfix/issue 239 inf vars

#### Summary:

Fixes issue #239 to allow variables with prefix `inf`.  

#### Intended Effect:

See summary.

#### How to Verify:

Added a unit test to `src/test/unit/gm` pointing to example grammar in `src/test/test-models/syntax-only/good_inf.stan`

#### Side Effects:

If anyone was using the undocumented (and unintended) behavior that `inf` would be read as an infinite floating point value, it will no longer work.

#### Documentation:

The change is user facing, but the old doc was correct.

#### Reviewer Suggestions:

Anyone.