Include WordComponentParser.hh in WordParserImpl.hh

To avoid compilation error due to incomplete type.
Add locale to parser::Environment

The locale is mainly used to define character type categories and
string-number conversion.
Make Predicate argument Environment const

A predicate should not modify the argument environment, so it should be
const.
Add isTokenDelimiter function
Add isVariableNameChar function
Add CLocaleEnvironmentStub class
Add StringParser class

StringParser is basically a clone of RawStringParser. StringParser just
returns a string so that the result can be used for many purposes.
Reimplement RawStringParser using StringParser

This commit is just a refactoring. No test cases added or modified.
Add AssignmentParserImpl template class
Add AssignmentParser class
Work around GCC bug

See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51629
Merge pull request #22 from magicant/feature/assignmentparser

Assignment parser implementation