.gitignore removed
core: actions_set_t

- NO_ACTION const added to represent non-valid action
- bug in define_new_action with actions incorrent redefinition fixed
- code_is_valid function added to check if passed action_code is valid
- sanity check in get_action_name added
- doxygen docs fixed
core: call_tree

- add_new_link_if_missing function removed
- action_set_t::NO_ACTION is used to represent non-valid action in root
- sanity check for action_code validity in add_new_link
core: updater

- DEFAULT_DEPTH renamed to DEFAULT_MAX_TRACE_DEPTH and is public now
- Don't throw exception in ~call_tree_updater_t, catch it and print to stderr
- sanity checks of action_code validity in start and stop
- get_trace_depth now returns intended depth(ignoring max_trace_depth)
- get_actual_trace_depth returns depth according to measurements stack
- get_current_node function added
- sanity checks of call_tree validity in action_code_is_valid
- more detailed exceptions are now thrown is action_guard_t
API

- react_call_tree_t and react_call_tree_updater_t replaced with
  react_context_t, which stores them.
- action_guard now stores unique_ptr<action_guard_t> inside to handle
  cases when react is now active by storing null pointer and ignoring
  start and stop commands.
- doxygen documentation added to react.h
- high_level example fixed to match new API
tests: Boost.Tests base added. CMakeLists.txt for tests added.
tests: actions_set test added
tests: call_tree test added
tests: call_tree_updater test added
cmake: tests are added to project. build options added.
tests: public_api test added