Added sort_indices_* functions
Added sort_indices_* functions to the manual
Added functions sort_indices_* to function_signatures.h
revised doc, section titles, and added examples for sort functions
refactor of sort_indices_* functions based on Bob Carpenter's https://github.com/stan-dev/stan/pull/564#issuecomment-35143532
Adjustments in rev and fwd diff for the refactoring of sort_indices_* functions
Merge branch 'feature/sort_index' of https://github.com/stan-dev/stan into feature/sort_index
templated container out rather than type of std::vector or Eigen::Matrix
A possible way to use to solve sign-compare warnings
Revert "A possible way to use to solve sign-compare warnings"

This reverts commit c4a46eb1ff58bcfe27c5fd799ea9092ede42ed16.
hopefully this fixes the sign errors
adding function sigatures tests for sort_indicies
fixed unsigned comparison warnings inside tests
Merge pull request #564 from stan-dev/feature/sort_index

Fixes #554.