#23 Improve query parsing by using an existing library

The parsing process of queries is not robust and providing queries not recognized may lead to errors or unexpressive error messages. For example, submitting the query ``most_used(macAddr);`` prints the error message ``An error occurred: near "None": syntax error``. Therefore I suggest re-implementing the query parsing functionality in `StatsDatabase.process_db_query` and `StatsDatabase._process_named_query`. This can be realized by using an existing library such as [pyparsing](http://pyparsing.wikispaces.com/).