JS int64 support

This adds signed/unsigned 64bit integer support.
Int64 values will be represented as arrays: [lowBits, highBits, unsignedFlag]. This format is taken from the widely used libraries Long.js and Google Closure, which therefore can be used to read/set those values.

I extended the currently included jspack library to support Pythons struct format q/Q codes. The new jspack is now referenced as sub-module and will be copied to the target location when generating the JS library with mavgen. My goal is to make this version of jspack available in the npm package repository so this copy operation will not be necessary anymore in the future.

All relevant changes are covered by unit tests. Test will now run on Travis.