Better content loading API (and docs)

[gudzpoz]

This PR adds a few API to facilitate content loading:

Buffers:

• ByteBuffer conversions via Lua::push
• Lua strings will be converted to ByteBuffer if Java typing requires so.
• One can now turn LuaValues into buffers (or vice versa).

Documentation:

• Javadoc for Lua::load is furnished, hopefully preventing misusages.

luaL_dofile equivalence:

• Add Lua::require.
• Encourage using Lua::require instead of relying on a filesystem.

Hopefully closes #207, #209 and #210