Include Emscripten CSpice.js build instructions

[gkjohnson]

Fix #21
Related to #5

• Update build instructions and include build script for the cspice.js file
• Lower the preallocated memory from ~400MB to ~100MB
• Adjust script and files new accommodate emscripten changes for at least v1.38.48

TODO

• Build a version based on the latest CSpice files
• Remove now-unnecessary functions (see Use UTF8ToString functions for reading pointers to strings. #21)
• Remove use of get_fs

Upcoming

• Use the latest version of Emscripten (Make build with latest version of Emscripten #32)
• Add flags for async WASM compilation (see onRuntimeInitialized callback) (Add flags to await asynchronous initialization #33)
• Add hooks for std output (Add event hooks for std in, out, err #19)
• Add ability to conditionally load different variants of cspice.js (asm.js 100MB version, WASM 100MB version, smaller build?) (Add multiple CSpice build options #34)
• Update docs to explain memory needs of library
• Consider using the MODULARIZE flag to enable multiple TimeCraft instances per page (Enable multiple TimeCraft instances with MODULARIZE=1 #35)