Add comparison operators for Keyword and Operator
Add static methods to create Keyword and Operator
Fix createTrie making dangling references

The previous implementation of Operator::createTrie was wrongly using a
initializer list of strings, in which temporary copies of the static
member constant strings are created and references to them are stored in
the operator objects in the trie.

The new implementation uses a initializer list of operator objects which
already contains valid references to the static members.
Add SpecificOperatorParser class
Add missing include in Token.hh
Fix segfault in TokenParser::parseWord

If the parseWord method is called for a second time, the word pointer
may already be nullified in the first call. We need to check if the
pointer is still non-null before dereferencing it, even if the word
parser is guaranteed to return a non-null pointer.
Add PipelineParser abstract class
Add PipelineParserImpl class
Merge pull request #52 from magicant/feature/pipelineparser

Implementation of pipeline parser