Improve the ELF part of LIEF

Major changes (features):
  * Enable adding multiple sections/segments - Executable (PIE or not), Library
  * Enable adding multiple dynamic entries (DT_NEEDED, DT_INIT etc)
  * Enable adding multiple relocations
  * Enable adding multiple dynamic symbols
  * Enable segment replacement

Major changes (API):
  * Getters Binary::get_*name*() has been renamed to "name()"
  * Binary::add(const DynamicEntry& entry) - To add an entry in the dynamic table
  * Section& Binary::add(const Section& section, bool loaded = true) - To add a section(s)
  * Segment& Binary::add(const Segment& segment, uint64_t base = 0) - To add segments
  * Segment& replace(const Segment& new_segment, const Segment& original_segment, uint64_t base = 0)
  * Binary's last_offset_section(), last_offset_segment(), next_virtual_address()
    to have information about offset
  * Binary's add_library(), get_library(), has_library() to handle
    DT_NEEDED entries

Other changes:
  * Binary::insert_content() - Use add(const Section&) or add(const Segment&) instead
  * ELF's DataHandler has been cleaned
  * Through LIEF::Section one can look for integers, strings, data
    within the section (see LIEF::Section::search,
    LIEF::Section::search_all)
  * Through LIEF::Binary one can get *xref* of a number (or address)
    see LIEF::Binary::xref function
  * To access to the Abstract binary in Python, one can now use
    the 'abstract' attribute. (e.g. binary.abstract.header.is_32)

Resolve: #83
Resolve: #66
Resolve: #48

api/python/ELF/objects/pyDynamicEntry.cpp

@@ -31,12 +31,18 @@ void init_ELF_DynamicEntry_class(py::module& m) {
 
   // DynamicEntry object
   py::class_<DynamicEntry>(m, "DynamicEntry")
+    .def(py::init<>(),
+        "Default constructor")
+
+    .def(py::init<DYNAMIC_TAGS, uint64_t>(),
+        "Constructor with " RST_CLASS_REF(lief.ELF.DYNAMIC_TAGS) " and value",
+        "tag"_a, "value"_a)
+
     .def_property("tag",
         static_cast<getter_t<DYNAMIC_TAGS>>(&DynamicEntry::tag),
         static_cast<setter_t<DYNAMIC_TAGS>>(&DynamicEntry::tag),
         "Return the entry's " RST_CLASS_REF(lief.ELF.DYNAMIC_TAGS) " which represent the entry type")
 
-
     .def_property("value",
         static_cast<getter_t<uint64_t>>(&DynamicEntry::value),
         static_cast<setter_t<uint64_t>>(&DynamicEntry::value),