Refactor methods to be contained in namespaced sub-classes / objects

Due to the amount of methods that exist for each Insightly API namespace, there are a large amount of methods that exist, especially for `Contacts`. Moving each to the respective namespace within InsightlyJS would help first-time users understand the the objective of what each method does, or on a more basic level, which asset within Insightly the method would affect. Additionally, if one is unsure about a specific method that is available, namespaced methods would provide an easy way of grouping related methods together.