Created asset installer, unit tests and updated documentation

Added a new type 'asset' with which you can define a specific location (relative to the composer.json) instead of the vendor-dir+package-name.

Possible vector with which to solve #524.

By adding an element `asset-dir` in the `extra` section of the package definition will the package install itself to a different location. This can be used to, for example, package twitter bootstrap and have it install onto a specific folder (such as /web).

Included in this PR is the source, an update to the documentation and a unit test.