[WIP] DOM API

[mbasso]

This PR aims to implement the new API described in #32.

Here is the list of tasks we need to fulfill:

• Update emscripten
• Update CI
• Update examples
• Update internals
• Add new examples that use the new API
• Tests
• Documentation

Here is the list of new functions we want to support:

inline int createElement(const char* sel);
inline int createElementNS(const char* sel, const char* ns);
inline int createTextNode(const char* text);
inline int createComment(const char* comment);
inline int createDocumentFragment();

inline void appendChild(const int parentElm, const int childElm);

inline void insertBefore(const int parentElm, const int newElm, const int referenceElm);

inline void setNodeValue(const int elm, const char* value);
inline void setText(const int elm, const char* value);
inline void setComment(const int elm, const char* value);

inline void setAttribute(const int elm, const char* attribute, const char* value);
inline void removeAttribute(const int elm, const char* attribute);

void setProperty(const int elm, const char* property, const emscripten::val value);
void removeProperty(const int elm, const char* property);

void setCallback(const int elm, const char* event, const Callback callback);
void removeCallback(const int elm, const char* event);

inline void remove(const int elm);

inline int parentNode(const int elm);
inline int firstChild(const int elm);
inline int nextSibling(const int elm);

inline emscripten::val getElement(const int elm);
inline int toElement(emscripten::val element);
inline void deleteElement(const int elm);