add sig location to symbols for improved symbol equality
Add IsEffectivelySameAs
Add more tests for signature symbols

Close #231.
IsExplicitInterfaceImplementation
Adjust unit tests

Close #232.
Merge pull request #242 from dungpa/sig-symbol-fix

Fix finding symbols declared in signature files
Merge pull request #243 from dungpa/intf-impl

Add IsExplicitInterfaceImplementation member to FSharpMemberOrFunctionOrValue
merge master