Bugfix/issue 835 fix exp precedence

#### Summary:

fixes parser so that it recognizes  exponentiation of indexed array items "a[i] ^ b[j]"

#### Intended Effect:

currently, parser is unable to parse "a[i] ^ b[j]" and compilation fails.  with bugfix, compilation will succeed, generated code will do the right thing.

#### How to Verify:

added stan model test/test-models/syntax-only/validate_exponentiation_precedence.stan
added test to test/unit/gm/parser/term_grammar_test.cpp

#### Side Effects:

none

#### Documentation:

exponentiation infix operator is already documented in Stan manual.

#### Reviewer Suggestions:

a parser fan.