Feature/issue 1060 build issues

#### Summary:

Adds additional dependencies for `bin/libstan.a` and `bin/libstanc.a` so they will automatically be rebuilt when header files change.

#### Intended Effect:

See summary. These targets didn't have dependencies on their respective header files (`.hpp`) which prevented them from being rebuilt automatically.

#### How to Verify:

1. Run a test, for example, `> ./runTests.py src/test/unit/gm/parser/get_lp_test.cpp`
2. Change the time-stamp on a header file needed for either `libstan.a` or `libstanc.a`. For example, `> touch src/stan/gm/grammars/whitespace_grammar.hpp`.
3. Run the test again.

Under the develop branch, the library files aren't rebuilt. With this fix, the library files should rebuild automatically.

#### Side Effects:

Added a `clean-deps` target and removed the `clean-models` target. If tests stop compiling, running `make clean-deps` should put everything back into the right state.

#### Documentation:

The makefile documents `clean-deps`. No other doc needed.

#### Reviewer Suggestions:

Anyone.