Set columns to unsignedBigInteger & Add foreign key to parent id

[charisoudis]

This pull request targets to two code improvements:

1. According to latest Laravel release increment columns are now by default big integer. Thus static::PARENT_ID and static::LFT - static::RGT columns should also be set to unsignedBigInteger by default.
2. Also, for better resulting db scheme parent id should be constrained with a foreign key constraint. This pull request add a foreign key constraint from static::PARENT_ID column to $idColumn = 'id' which is passed as an optional parameter to nestedSet() macro function.