Ensured that the __has_{builtin,feature,extension}() macros are
defined as no-ops in <cpr/prelude.h> in case the compiler doesn't
support them, making them safe to use.

See: http://clang.llvm.org/docs/LanguageExtensions.html#feature-checking-macros
Implemented the cpr_vector_alloc() and cpr_vector_free() functions.
Implemented the cpr_vector_init() and cpr_vector_dispose() functions.
Implemented the cpr_vector_size() function.
Implemented the cpr_vector_empty() function.
Implemented the cpr_vector_clear() function.
Commented out all unimplemented modules from cpr_module_names.
Implemented the cpr_vector_push_back() function.
Implemented the cpr_vector_pop_back() function.
Implemented the cpr_vector_at() function.