Feature/issue 1060 build issues

[syclik]

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.

[stan-buildbot]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
http://d1m1s1b1.stat.columbia.edu:8080/job/Stan%20Pull%20Request/14/Test PASSed.