New ASTMatchers and enhancement to hasOverloadedOperatorName

Added two new narrowing matchers:
* hasMethod: aplies a matcher to a CXXRecordDecl's methods until a match is made
  or there are no more methods.
* hasCanonicalType: applies a matcher to a QualType's canonicalType.

Enhanced hasOverloadedOperatorName to work on CXXMethodDecl as well as
CXXOperatorCallExpr.

Updated tests and docs.

Reviewers: klimek, gribozavr