- added Setting.lookup() and config_setting_lookup()
- documentation updates
- version bump

To address hyperrealm/libconfig#3, I have added a SettingIterator class
and begin() and end() methods on Setting so that lists and arrays of
Setting can be iterated through using C++ style iterators.

There is an iterator as well as const_iterator defined for the class.

To address hyperrealm/libconfig#4, `Config::operator[](const char*)` and
`Setting::lookup(const std::string& key)` have been added.

Note: `operator[]` had to remain as `const char*` to avoid ambiguities
      in some situations.

To address hyperrealm/libconfig#1, the cast operator from Setting to
const char* has been removed. As a result, lookupValue() taking a const
char* has also been removed as it uses the cast operator.

This change allows code that assigns directly to a std::string or a
const char* to compile without errors. No change to existing code should
be required.

To address hyperrealm/libconfig#2, a constructor has been added to the
header and source file.