base: ensure Element::Error inherits from tlv::Error

Adding static asserts to ensure (Name|name::Component|Interest|
Selectors|Exclude|KeyLocator|Data|MetaInfo|Signature|SignatureInfo)
::Error are subclasses of tlv::Error.

This commit also adds WireEncodable and WireDecodable concept checks
to Name,name::Component,Interest,Selectors,Exclude,KeyLocator,Data,
MetaInfo,SignatureInfo types.

This commit also moves definition of Selectors method into .cpp.

refs #1983

Change-Id: I15220b93437d4a624ae09df66defc91ceac2386c
diff --git a/src/signature.cpp b/src/signature.cpp
index 5ec68d5..723478c 100644
--- a/src/signature.cpp
+++ b/src/signature.cpp
@@ -23,6 +23,10 @@
 
 namespace ndn {
 
+BOOST_CONCEPT_ASSERT((boost::EqualityComparable<Signature>));
+static_assert(std::is_base_of<tlv::Error, Signature::Error>::value,
+              "Signature::Error must inherit from tlv::Error");
+
 Signature::Signature(const Block& info, const Block& value)
   : m_info(info)
   , m_value(value)