Issue #49
Classes for holding statistics of tags implemented.
Issue #49
1) Copy constructor for TagStatisticsCollection implemented.
2) `get' method of TagStatisticsCollectionIterator no longer reutrn pointer.
   It was a wrong thing to do.
Issue #49
Method `get_tags_statistics' implemeted in Database layer.
Issue #49
`get_tag_statistics' added to the Engine and Front End layers.
Unit Tests added.
`front_end/tag_statistics.h' added to the headers that should be installed.
`--all-tags' added to CLI options.
Code clean up.
A file to guid contributers.
New notes about Unit Test, Commit Messages, and Trailing Spaces.
Merge branch 'master' of https://github.com/aidin36/tocc
merge with Aidin's master